So you would need about a 5% oxygen atmosphere in Equestria. That’s because if it was 20% it’d reach toxic levels for humans (and presumably ponies, too).

So, in Equestria, Mario Kart 8 karts could glide longer and higher once they touch blue ramps with arrows

@redweasel  
They could also be using very small pieces of wood.

god you people, obviously they use inflatable wood sheesh it’s not rocket science

@Background Pony #58A6  
Some theories say that pegasus magic allows the water of the clouds to turn into non-newtonian dilatant fluid, the more they press, the harder they get. Maybe that works on the air too, as both are fluids. The magic may concentrate in pegasus hooves and wings only.

@Wiimeiser  
Skin is actually the heaviest tissue as long as you count the subcutaneous fat as something separate. If however you count the subcutaneous fat as part of it, it’s considerably less dense.
 
 
@Background Pony Number 17  
That’s long been my sense of it as well. Horns, wings, or hooves, each type of pony has a magic-channeling organ that accounts for their differences from real-world ponies. And it’s the magic, not the physical properties of the organ, which accomplishes it. Wingpower to break the sound barrier simply doesn’t exist without magic. Much less, why breaking the sound barrier would have optical effects (aside from Schlieren scintillation if you have the right lens setup to see it). Or why a couple pegasi can generate Breezie-aiding winds of unidirectional but variable speed, while maintaining a constant altitude and zero velocity themselves.
 
And atmospheric density doesn’t explain why pegasi can make things out of clouds or walk on them. If the clouds are dense enough to buoy up pegasi by only their hooves, then the clouds must be around the density of mercury if not more (and/or pegasi must be unusually light, like birds; and/or their hooves must strongly repel the clouds, like the feet of a pond skimmer insect). Which means the clouds would fall like stones through a mere 5X denser atmosphere, and probably buoy up most rocks right out of the ground too after they soaked in. And if earth and unicorn ponies can’t walk on mercury clouds, non-pegasus ponies have to be denser than mercury, which puts them somewhere in the vicinity of uranium or osmium, if not quantum degenerate stellar remnants. None of which makes any sense, so magic has to explain the interactions between pegasi and clouds too.
 
So yeah, it has to be magic. But speculating on the consequences of it being due to density is more fun in the short term.

I’ve always assumed pegasi fly by nullifying gravity and generating thrust magically. Their wings are mainly to provide steering and as a focus for their flying magic (which explains why some pegasi are better flyers than others).
 
Balloons and airships, of course, float due to being filled with less dense gas, either hot air, hydrogen or helium. Of course, unicorns could enchant the gas bag’s contents, magically making them less dense than air. From what we’ve seen in ths show, balloons are steerable, implying they’re moved by magic as well as buoyancy.

The atmosphere would have to be thick enough to allow termites the size of Apatosaurs, keratin weight be damned. IRL a human would need wings the length of a parking space, each, to fly, but such wings would be too large to function. Assuming the issue there is a circulatory one, it’s possible our hearts will need to be the size of a small pony and strong enough to launch six Playstations into space with one beat, which would probably be loud enough to blow our hearing. But that would only worsen things. It’s like two people being taller than each other. IIRC muscle is the heaviest tissue in the body, so simply giving birds the ability to swallow without gravity might make them too heavy to fly.

@Background Pony #FA03  
Doesn’t mean you can’t have some fun trying to apply science over it. Superhero comics use laws of physics for action sequences and plot points all the time.

The gravity note might not apply simply because as far as we can tell, Equestria is a geosynchronous planet with the Sun orbiting it. Just a slight note.

Using feets, inches and Kilograms? What?  
If you’re going to use a measure system, be it US or international, at least use the same one for both scales.
 
And how to debunk that theory: If the atmosphere is x5 times thicker, clouds should be in the stratosphere, as water as we know it has the same desity in this universe of (in your units) 1 Kilogram per 3.531467 cube feet, instead of ~200 feet or so as they do in the show.  
Another one: Having such atmosphere density, air friction would be HORRIBLE. RD would burn even before getting to match 1. And storms and wind would be devastating for the living beings on the surface. Not to mention more dense means more heat that can he absorved, and visibility would be much worse, so plant life would suffer.
 
Fun fact: if they breathe oxygen (they do because they can lit up fires and use carbon engines)and their biology is similar to ours, more oxygen means more cancer. If the atmosphere is that dense, ponies might have a god damn great immune system to combat cancer, which might explain how can they use their mouth so much without getting infected.

“However, there is an explanation”
 
Yes: It’s a children’s cartoon show founded on magic
 
[/topic]

It would however imply that Rainbow Dash’s speed may not be what we thought. High atmospheric density, if due to pressure, would raise the speed of sound. If due to heavier molecules, it would lower the speed of sound. So if they live in a nitrogen-oxygen atmosphere, similar to ours but just 5 atmospheres of pressure (which would probably be survivable; that’s roughly the air SCUBA divers breath at 120 feet, which is just a little past the limit where diving can be safely accomplished with compressed air alone rather than an exotic gas mixture), Rainbow would have to be going even faster than what we consider the “speed of sound” to exceed her atmosphere’s Mach 1 and create a sonic rainboom. (As an aside, it could explain a lot about Equestria. At that pressure oxygen and nitrogen can be hallucinogens.)
 
However, if they live in an atmosphere that’s mostly, say, sulfur hexafluoride, the speed of sound would be a lot lower (only about 44% of our Mach 1), and Rainbow Dash wouldn’t have to go particularly fast to create a sonic rainboom.

Counterpoint: if Equestria-Earth’s (or whatever the planet’s name is) atmosphere was significantly thicker than Earth’s, then all winged animals would have the same disproportionately small wings that pegasi do. As it is, they have the same sort of proportions as ours do, suggesting an atmosphere of similar density.
 
As for airships:  
 
The proportions don’t seem that far off, especailly if you replace the steam engine with a significantly lighter (and much more powerful) gasoline engine.
 
I generally tend to assume that pegasus and alicorn wings aren’t really lifting surfaces at all, but instead produce thrust for both lift and forward movement through some sort of magic. This would also explain haw they can hover with such slow wing movements and how Tirek was able to rob them of their ability to fly in the Season 4 finale.

Fun fact: this is the Derpy Hooves picture number 33333