>>58188869Which isn't surprising in the least, here's why:
Aerial Ace's Japanese name (which often does a better job of conveying the nuance of why certain Mon learn certain things) is Tsubame Gaeshi which roughly translates to Swallow's Return. This can either be interpreted literally as a bird flying quickly and returning to it's original spot, or it can be taken as a reference to a sword technique of the same name and as such, it is a slashing move.
This means the Pokemon learning it would need to either be physically capable of or have a design based upon something capable of bird-like/winged locomotion or slashing something with claws, pincers or some other kind of sharp thing.
This would explain why Gyarados specifically cannot learn it, Gyarados' form of flight doesn't involve wings and it's only sharp appendage is it's fangs which would be used for biting moves. Many Pokemon who learn it who don't have wings/an obvious appendage for flight are at least in the Flying egg group too, such as Togepi.
Being fair though, there are some outliers. Mantine could MAYBE make sense if interpreting it's wing-like fins as wings and it's gliding after leaping out of water as a type of "flight". That being said there is one Pokemon who has absolutely no excuse: Bruxish. Nigga how the FUCK can Bruxish learn it?
But yeah, some moves have some weird distribution. Like the iconic instances of things like Primeape's tailless Iron Tail or Wooper and Gastly learning fucking Ice Punch.
My personal favourite is that Cryogonal can learn Attract despite being Genderless and as a result the move cannot succeed.
