I used a female Mudsdale named Applejack, dynamaxxed her and used Rock-Type move to finishes off the Champion’s Charlizard.

Now I feel complete about this image.

Discord is a Normal type because it’s the last thing you’d expect. He has only one attack, Metronome.

@evil_eric  
Pinkie and Discord normal types, woah nelly. I guess in Equestria there would be a majority of ground, flying, and psychic types.

@Tsitra360
 
I always thought of them this way:
 
Twilight: Psychic/Flying  
Rainbow: Electric/Flying  
Applejack: Ground/Fighting  
Rarity: Psychic/Rock  
Fluttershy: Grass/Flying  
Pinkie: Normal  
Spike: Dragon/Flying
 
Here’s some more Ponymon, because frosting.
 
Celestia: Fire/Flying  
Luna: Dark/Flying  
Cadance: Fairy/Flying  
Shiny: Psychic/Steel  
Starlight: Psychic  
Sunset: Psychic/Fire  
Trixie: Psychic  
Discord: Normal  
Apple Bloom: Normal  
Sweetie Belle: Normal  
Scootaloo: Normal  
Sandbar: Normal  
Gallus: Normal/Flying  
Yona: Fighting  
Silverstream: Water/Flying  
Smolder: Dragon/Flying  
Ocellus: Bug/Fairy  
Chrysalis: Bug/Dark  
Sombra: Ghost/Dark  
Tirek: Dark/Dragon  
Maud: Rock  
Derpy: Water/Flying

I think of AJ as Ground type, and Dash as Flying type.
 
Twilight: Psychic/Flying Type  
Fluttershy: Grass/Flying Type  
Pinkie: Ghost Type (explains why she can appear anywhere)  
Rarity: Rock Type. (Gems)  
Spike: Ironically not a Dragon Type, but Fire

Applejack I choose you

@Beau Skunky  
AJ is essentially a Mudsdale in my mind, pure ground, so her STAB attacks wouldn’t do any damage, but she’s not being countered by Dash in any way, and be slower than a wet weekend, but that’s alright, she’s tanky af, and she still has access to moves like Heavy Slam, so the heavier she is, the harder she hits (and at 920kg of pure equine muscle, that’s gonna hit like a train), Stomp, and Mega kick. So she can still dish out a lot of damage, and take it due to the Stamina ability raising her defense every time she gets hit.
 
Going by the fact that’s a Galarian league pitch, they’re probably in Wyndon doing the semi-finals for the league challenge, so going by the fastest flying type available in Galar, that would mean Dash is a Noivern, Flying/Dragon  
Assuming Applebloom left Counter on AJ, she’s actually got an advantage in the battle, because flying isn’t resistant or weak to fighting types, and neither is dragon, she has a pretty good opening to just spam counter, dealing back double the damage Dash deals out with any physical attack she uses, and considering physical is Noivern’s preferred attacking stat. All she’d have to watch out for is Boomburst, but AJ’s sp.def should be appreciable enough to mitigate a lot of that damage, it would still hit harder than anything else Dash might have in her movepool, even without the STAB.  
Though Dynamaxing does change the way moves work, so she loses counter while dynamaxed, becoming max knuckle, which removes one of her best offensive tools because it caps out the base power at 75, but a max strike from mega kick would still work well there.
 
tldr, Appebloom has the advantage normally, but Dynamaxing evens out the playing field because neither side has any access to moves that give them a leg up over the other, type advantage wise. Granted, in previous generations Noivern could learn water pulse from a move tutor, but a move tutor for that move doesn’t exist in SwSh, leaving Dash with a varied type array of moves, but none that can super effective AJ. AJ’s movepool is a lot more limited, but it’s much the same, she can hit neutral.. and that’s about it.
 
That’s just my take on the whole battle using assumptions, knowledge of type matchups, stats and movepools. All it would take is for Tsitra to add what types they are to the description to totally throw my theorising here out the window.

@Beau Skunky  
You could make the argument for Ground-type. But I feel that she would still be either Grass- or Fighting-type, both of which are weak to Flying. Still a disadvantage.

I’m secretly hoping that there’s gonna be a picture coming from Scootaloos and Dashs POV now.

@ArtColter  
Being an earth/mud pony, shouldn’t that make her an earth type Ponémon?

AB is at a distinct disadvantage. Applejack is a Grass/Fighting type, which is 4x weak to Rainbow Dash’s Flying-type moves.

It was inevitable.

Go! Big Apple Pony!

Ponémon!
 
This is very epic, and I like how Apple Bloom is her trainer.