Pokedex entries are psuedo-scientific at best but locomotion is a sorely abused biological concept in a lot of Pokémon designs. Other than Garchomp flying around at “jet plane” speeds*, what are some the most egregious examples in your opinion? For me, the two other worst offenders are A) Escavalier, which somehow “fl[ies] around at high speed” – I like to pretend it actually hovers on rapidly flapping insect wings, too fast for human eyes to see – and B) the Diving Pokémon Lugia – I cannot explain how a benthic organism can use those same large flippers to break the surface and move its body through the air using anything other than “psychic force propulsion”.
*in my mind, Garchomp has a modified swim bladder that has evolved allowing it to float in the lighter-than-water fluid that is air, but I still can’t imagine how Garchomp achieves lift to propel itself through the air.
Escavalier always really bothered me too, just because of how slow the damn thing actually is in the game; you see the same kind of thing with Donphan, Vikavolt, probably some others (EDIT: however, see here on what “speed” and other stats actually mean). Honestly, I don’t even care that it doesn’t have wings, because I can totally imagine it just, like, levitating on magnetism or something – although of course the bastard thing doesn’t learn Magnet Rise either. Speaking of Magnet Rise, though, Jim the Editor found a fantastic one, which is that Electrode can apparently become so bloated with electricity that it floats and drifts along on the wind.
All that said, though, off the top of my head I can’t think of anything that tops Garchomp being able to fly – and I will note here that it can’t actually learn Fly, which would have been a very easy move to give it in a practical sense, and not a serious game balance threat. That’s actually what bugs me here, the lack of consistency, that Garchomp in the game doesn’t do anything that makes it seem like it should be able to fly. ‘cause, like… the physics of flight clearly don’t work for a whole lot of Pokémon; I don’t really believe Charizard could fly with wings that size, and Dragonite sure as hell couldn’t. But Charizard’s clearly a traditional European dragon, and they’re supposed to fly whether the physics works or not, and Dragonite’s all mystical and sacred and stuff. Likewise, I’m not bothered by just about anything Lugia does because Lugia clearly is extremely magic. There’s realism, and there’s verisimilitude, y’know?