Found this gem on Reddit. It was made by a guy who isn’t a brony, but wanted to please their pegasister friend who is a fan of Discord’s humour and tendency to make references. He watched every episode with Discord in just so that he could make her an image filled with as many references as he could find for her birthday.
 
He also shared a Google Doc he made that explains what he thinks every reference/trope was as he says that some were more difficult to understand than others. The guy put in more effort than most actual bronies, and all for his friend. Below are all the descriptions from the shared Google Doc.
 

 
REFERENCES FROM LEFT TO RIGHT
 
Row 1
 
The Return of Harmony: John de Lancie, Discord’s voice actor, is probably best known for playing Q in Star Trek: The Next Generation (Discord’s inspiration). Q is an extremely powerful extra-dimensional being of unknown origin who uses his seemingly limitless power to play practical jokes on others for his own amusement. Both characters have Machiavellian personalities and tend to snap their fingers to appear nonchalant while warping reality.
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. Discord, Twilight and Princess Cadance pay homage to the Harry Potter franchise. Princess Cadance is a Ravenclaw student, Twilight is Harry Potter and Discord appears to be Severus Snape.
 
Make New Friends but Keep Discord: One of the numerous references made during Discord’s painful comedy routine. Discord is dressed up as Leo Anthony Gallagher Jr. Gallagher is a comedian who was known during his career for smashing food, typically watermelons, as part of his prop comedy act.
 
Discordant Harmony: From what little I’ve seen of the show, Discord appears to be one of the only characters who is allowed to acknowledge modern technology. When the Discord clones are judging the original Discord’s costume changes, four of them are seen using simplified versions of an iPhone.
 
The Big Mac Question: In order to convince Big Mac and Spike to share the big secret with him, Discord claims that the three of them are amigos. At this time, the three of them are briefly seen wearing costumes that pay homage to the three protagonists of the 1986 comedy film ¡Three Amigos! (Lucky Day, Dusty Bottoms, and Ned Nederlander).
 
Best Gift Ever: As a last resort, Rainbow Dash buys Fluttershy a candle for Christmas, but it is then revealed to be Discord in disguise. Probably a reference to Lumière, the talking candlestick from Disney’s 1991 film Beauty and the Beast.
 
 
Row 2
 
What About Discord?: When Discord flies alongside Rainbow Dash, he travels fast enough to create a plaid stream. This is a reference to the satirical comedy film Spaceballs where travelling at ludicrous speeds creates a similar effect and is referred to as ‘going plaid’.  
Celestial Advice: During a celebration party, Discord wears a pair of Groucho glasses. These fake glasses were created to give the wearer the likeness of the famous comedian Groucho Marx.
 
The Return of Harmony: During his first episode, Discord does the Running Man dance whilst in a stained glass window. The Running Man dance originated in the 1980s and was popularised by Janet Jackson thanks to the music video of her 1989 hit Rhythm Nation.
 
The Return of Harmony: Discord loves unusual weather and his chocolate rain is brought up several times in the few episodes of the show that I have seen. May be more than just a manifestation of Discord’s chaotic nature as the concept of chocolate rain became an internet sensation when Tay Zonday released his iconic song back in 2007.
 
Make New Friends but Keep Discord: When getting ready to send Fluttershy’s new friend to an alternate dimension, Discord uses a long-stemmed microphone similar to the ones famously used by Bob Barker of The Price Is Right fame. Also, Discord’s description of the alternate dimension pays homage to the way game show hosts describe exotic trips to contestants.
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. Discord is briefly seen rubbing a magic lamp. The familiar design of the lamp makes this a clear reference to Disney’s 1992 film Aladdin.
 
 
Row 3
 
Discordant Harmony: Discord considers dressing up as King Henry VIII of England when deciding on what to wear to his tea party with Fluttershy. The head clone turns down the idea because he has no class.
 
The Return of Harmony: Discord’s reign results in a stampede of rabbits with freakishly long legs. These rabbits seem to have been inspired by Salvador Dali’s famous painting The Elephants. After all, like Discord, Dali was defined by his preference for the surreal.
 
To Where and Back Again: In order to prove that he is who he says he is, Discord demonstrates his ability to warp reality. One of his tricks involves transforming Trixie’s wagon into a purple pumpkin carriage. A chaotic version of the famous spell from the folktale/1950 Disney film Cinderella.
 
Best Gift Ever: Whilst ‘helping’ Rainbow Dash with her Christmas shopping, Discord adopts the appearance of Snoopy’s World War l Flying Ace persona from the Peanuts comic strip.
 
The Break Up Break Down: Another example of Discord acknowledging technology not typically seen in the show. When Spike claims that Discord doesn’t attend his tea parties with Fluttershy for the tea itself, Discord disagrees by listing all the different types of tea that he enjoys drinking. He does this by starring in his own stereotypical cooking show.
 
What About Discord: During the episode, Discord frustrates Twilight by impersonating famous television artist Bob Ross on two separate occasions. Bob Ross was known for his permed hair and calm on-screen persona.
 
 
Row 4
 
Dungeons and Discords: Discord decides to spice things up by bringing Spike’s game to life and becoming the Dungeon Master. His appearance as a menacing face in the clouds references two tropes frequently used in fiction. One trope is when a character communicates through clouds (The Lion King/Monty Python etc). The other trope is when an evil onlooker ‘watches’ the actions of the hero from afar (various video game box art etc).
 
What About Discord?: When thinking about how they can accurately recreate their previous outing, Discord dresses as Doc Brown from the Back to the Future franchise. After all, Twilight claims that they need to recreate the past for ‘scientific’ reasons.
 
A Matter of Principles: Discord tries to sabotage Twilight’s school by barging into a classroom whilst dressed as Steve Buscemi’s character from the satirical sitcom 30 Rock. Discord recreates the famous ‘How do you do, fellow kids?’ meme by asking the pupils ‘How do you do, fellow creatures?’.
 
The Ending of the End: When Twilight refuses to let her friends fend off the antagonists alone, Discord grabs her and yells ‘Fly, you foal’. A reference to Gandalf the Grey’s famous last words before losing his life to a Balrog in the 2001 fantasy film The Lord of the Rings: The Fellowship of the Ring.
 
Discordant Harmony: Discord describes himself as ‘showy’ whilst donning a Brazilian festival costume. A shout out to the extravagant Carnival in Rio de Janeiro festivals that are held every year.
 
Discordant Harmony: Discord decides to change his hairstyle to try and make Fluttershy more comfortable at his tea party. Some of his options are two of the most famous real world hairstyles in history. The first hairstyle is the iconic ‘Elvis look’ whilst the second is the ‘Beehive look’ which was developed in 1960 by Margaret Vinci Heldt (not the woman in the picture).
 
 
Row 5
 
Make New Friends but Keep Discord: When Discord decides to invite the Smooze to the gala, they both turn up dressed as one of the main protagonists from the 1994 comedy film Dumb and Dumber (Lloyd Christmas and Harry Dunne).
 
Dungeons and Discords: Discord eventually begins to enjoy playing whatever it was they called their version of Dungeons and Dragons. He uses his powers to turn everyone playing into some sort of reference. Discord and Spike become Legolas and Gandalf from The Lord of the Rings, respectively. Big Mac becomes a warrior wearing the iconic horned helmet from the game Skyrim. Rainbow Dash’s costume is a lighter variant of Garret’s from the rebooted Thief games and Pinkie Pie appears to be a Gallic Strongman since she bears a resemblance to Obelix Obelodaliges from the Asterix comics.
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. Discord leans back in a chair before pouring water over himself. One of the billions of references to the 1983 drama film Flashdance that have been made over the years.
 
Make New Friends but Keep Discord: One of the numerous references made during Discord’s painful comedy routine. Discord spends part of his act dressed in a suit. When he fails to make the audience laugh, he turns to the camera and comments on what a tough crowd they are. This a nod to the comedian Rodney Dangerfield, who was known for his suit, self-deprecating humour and intense facial expressions.
 
Twilight’s Kingdom: Only know this one because I eventually resorted to looking up any references that I missed. Instead of simply vanishing, Discord leaves one scene by getting on the same electric scooter that the main protagonist of Littlest Pet Shop uses (another show made by Hasbro Studios).
 
Best Gift Ever: In one scene of the episode, Discord gets a face full of snow. When he picks his head up off the ground, he looks like someone who doesn’t exist in their universe…or ours.
 
 
Row 6
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. When singing about lozenges, a pony in lederhosen playing the sousaphone can be seen in the background. This is a parody of the Ricola adverts which are known for featuring the alphorn playing mountain dwellers of the Swiss Alps.
 
 
Make New Friends but Keep Discord: Whilst trying to find someone who would be willing to take him to the gala, Discord hides inside a cardboard box. When Pinkie Pie immediately identifies him, a red exclamation mark appears over the box. A clear reference to the Metal Gear Solid franchise of stealth games where hiding in a cardboard box can help stop enemy guards from going all red exclamation mark on you.
 
Dungeons and Discords: Discord takes Spike and Big Mac to a jazz nightclub in order to try and get away from their roleplaying game. Whilst there, they start playing poker with some dogs, as in the Dogs Playing Poker painting series by C. M. Coolidge.
 
Discordant Harmony: Discord finally settles on an outfit he deems acceptable for his tea party with Fluttershy. His normal outfit is that of Mr Rogers, beloved host of the show Mister Rogers’ Neighbourhood.
 
What about Discord?: This episode is named after the 1991 comedy film What about Bob? and Discord dresses up as the main protagonist, Bob Wiley, in one of the last scenes. Also, said scene sees Discord with a miniature Smooze in a jar hanging around his neck. This small detail references the part of What about Bob? where Wiley puts his goldfish in a jar and hangs it around his neck.
 
To Where and Back Again: Discord attempts to charge the lair where the main protagonists are being held prisoner by riding a flying pig, wielding a lance and shouting ‘For Fluttershy’. This is an allusion to the scene where a mounted Peter Pevensie shouts ‘For Narnia and For Aslan’ before charging into battle in the 2005 fantasy film The Chronicles of Narnia: The Lion, The Witch and The Wardrobe.
 
 
Row 7
 
A Matter of Principles: One of the ways Discord disturbs a lesson at Twilight’s school is by answering a phone call with a banana. May just be another way of representing Discord’s chaotic wackiness, but may also be a reference to the song Bananaphone by Raffi. Rather fittingly, Discord’s previously mentioned Steve Buscemi costume includes a similar red hat to the one Raffi wore when advertising said song.
 
Twilight’s Kingdom: Discord betrays the heroes and starts capturing ponies so that Lord Tirek can drain them of their magic. One scene shows him catching himself a group of ponies by lassoing them with a fishing rod. During this scene, Discord is dressed up as Sam Quint from the 1975 thriller film Jaws. Quint was a seasoned shark hunter who was hired to assist the main protagonists during the events of the film.
 
Twilight’s Kingdom: Before joining forces with Lord Tirek, Discord is originally tasked with stopping him. The scene where they first meet sees Discord dressing up as a policeman before dodging Lord Tirek’s attack by splitting his head in half. Possibly just a coincidence, but this moment draws a parallel to one of the most iconic scenes from the 1991 action film Terminator 2: Judgement Day. The scene in question has the main antagonist, the robotic T-1000, have his head momentarily blown in half whilst disguised as a policeman.
 
Three’s A Crowd: Discord feigns illness and tricks Twilight into going on a journey to find him a ‘cure’. As he claims to be too weak to travel by himself, Discord has Twilight and Princess Cadance pull a chariot that replicates the one owned by the main antagonist of the 300 Spartans action films, King Xerxes.
 
Best Gift Ever: Rainbow Dash is guilted by Discord into inviting him to the Christmas party. Discord responds to the invitation by wearing Cousin Eddie’s outfit from the 1989 comedy film National Lampoon’s Christmas Vacation. Cousin Eddie is a jovial redneck and a recurring character in the Vacation franchise.
 
Three’s A Crowd: When Discord sees how far Twilight was willing to go to get him a ‘cure’ for his feigned illness, he celebrates and moves off-screen with a Vaudeville dance. Vaudeville was a form of light entertainment that originated in France before becoming incredibly popular in America and Canada between the 1890s and 1930s.
 
 
Row 8
 
What About Discord?: Discord summons a ticker tape machine when trying to recreate his previous outing with Twilight’s friends. The ticker tape was the earliest electrical dedicated financial communications medium, used to transmit stock price information over telegraph lines between around 1870 and 1970. Discord’s nineteenth century getup references the time period when the ticker tape machine was invented.
 
Dungeons and Discords: Discord arrives at the guy’s night by bursting through a banner as a basketball player. Having a thirteen on his basketball jersey references both the stereotypical chaotic nature of the number and the basketball player Wilt Chamberlain, who famously wore that number throughout his career.
 
Three’s A Crowd: When giving Twilight and Princess Cadance directions of where to go to find the ‘cure’ to his feigned illness, Discord is wearing the outfit of antihero Raoul Duke from the 1998 black comedy film Fear and Loathing in Las Vegas.
 
What About Discord: Discord disappears in front of Twilight, but not before running on the spot fast enough to cover Spike in a pile of dirt. This is a reference to the wheel o’ feet trope frequently seen in older cartoons, typically for comedic effect. One of the most notable examples is the Road Runner from the Looney Tunes franchise.
 
Twilight’s Kingdom: Whilst explaining his task of capturing Lord Tirek to Twilight and her friends, Discord references George C. Scott’s portrayal of General George S. Patton in the 1970 war film Patton. At the same time, Discord references General Douglas MacArthur by ‘smoking’ his iconic corncob pipe.
 
Make New Friends but Keep Discord: The cane Discord takes to the gala includes the Symbol of Chaos. The symbol originated in Michael Moonrock’s Eternal Champion book series and is known for being adopted by the Warhammer franchise.
 
 
Row 9
 
Make New Friends but Keep Discord: One of the numerous references made during Discord’s painful comedy routine. Discord briefly impersonates Eddie Murphy by donning his red leather jacket and gold necklace from the 1983 Eddie Murphy Delirious stand-up comedy special.
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. Discord sees a star shining outside the window and makes a wish. A recreation of the woodcarver Geppetto’s famous song from the 1940 Disney film Pinocchio.
 
Twilight’s Kingdom: Discord reveals to Twilight’s friends that he has betrayed them by locking them up in a cage. When Lord Tirek asks whether he got them all, Discord responds with ‘And her little dragon, too’. This is a nod to the Wicked Witch of the West from the 1939 fantasy film The Wizard of Oz, who in one scene tells Dorothy ‘I’ll get you, my pretty, and your little dog, too’.
 
The Summer Sun Setback: As the celebration begins to break down, Discord appears as a holy figure to offer encouragement to Twilight’s friends. It’s possible that he’s masquerading as a mere angel, but his blue sash draws parallels to one of the common ways in which Jesus Christ has been depicted in art over the years.
 
Three’s A Crowd: After moving off-screen with a Vaudeville dance, Discord slides back on-screen whilst wearing some underwear and a pair of socks. This is a reference to a scene from the 1983 comedy film Risky Business. The scene in question involves the protagonist of the film, Joel Goodson, dancing in his underwear and socks by sliding across the floor of his home. People who reference the dance often wear a pair of sunglasses despite Goodson not wearing any in the original scene, Discord being no exception. This mistake is common because Goodson is known for wearing sunglasses during other parts of the film.
 
What about Discord: Discord reflects on what the ‘old him’ would have done if put in his current situation. The design of the ‘old him’ references the villain Snidley Whiplash from the Dudley Do-Right of the Mounties segments of the animated television series The Rocky and Bullwinkle Show that ran between 1959 and 1964. Whiplash is often associated with the famous gag of the villain tying their victim to a railroad track. However, this gag actually became well established decades before Whiplash was first introduced.
 
 
Row 10
 
Best Gift Ever: Discord tags along to help Rainbow Dash catch a new pet for Fluttershy. When she tells him to be quiet, Discord dresses her and himself as Storm Shadow and Snake Eyes, respectively. These two characters are ninjas from the G.I. Joe franchise. Also, the music that plays during this scene alludes to the theme of the original G.I. Joe.
 
Make New Friends but Keep Discord: When getting ready to send Fluttershy’s new friend to an alternate dimension, Discord shows a pony version of Grant Wood’s American Gothic of 1930, one of the most familiar paintings of twentieth century America. The painting has gone on to be widely parodied in American popular culture.
 
Twilight’s Kingdom: Discord approaches Twilight and her friends by slowly descending from the sky. His umbrella, hat, scarf and magic bag are all references to Julie Andrews’ portrayal of Mary Poppins from the 1964 Disney film of the same name.
 
Dungeons and Discords: Discord takes Spike and Big Mac to a jazz nightclub in order to have a ‘REAL guy’s night’. Discord wears a zoot Suit at the nightclub, a style of clothing that became popular in African-American, Latino, Italian American, and Filipino American communities during the 1940s. Also, this scene appears to be a reference to the 1994 comedy film The Mask. Putting on the titular wooden mask allowed the protagonist of the film, Stanley Ipkiss, to follow Discord’s example and cartoonishly alter himself and his surroundings at will. Whilst adopting his superhero persona, Ipkiss dons a zoot suit and spends an evening in a jazz nightclub similar to the one Discord takes Spike and Big Mac to during the episode.
 
Three’s A Crowd: One of the numerous references made during Discord’s musical number. Discord holds up his empty glass of water in a triumphant fashion against a space background. A recreation of the poster for the original Star Wars film of 1977.
 
The Summer Sun Setback: Discord helps fix the sabotaged celebration by pulling up the bottom edge of the screen like a curtain and shoving one of the unwanted clouds through. Pulling up the bottom edge of the screen exposes a mass of static. A fourth wall break that references noise, a static pattern displayed when no transmission signal is being obtained by the antenna receiver of the television in question.