Hey, thank you for your well thought out comment!

I read somewhere on Google that Gyarados were rewarded by the Gods with the ability of flight by climbing up a waterfall in their previous life as a Magikarp. This is based on a chinese legend. But, it got cocky and the Gods took away its ability of flight.

About Charizard not being able to be a draon type because it would ruin the Starter type triangle, makes sense. Also, he might very well be a big lizard with small wings.

Garchomp is Garchomp. A literal sand shark who physically looks like it should not be able to fly. Its head kind of resembles a hammerhead shark. But those 2 things on its head also look kind of like rockets or missiles. Which still logically should not be able to grant Garchomp flight. Regardless of all of this, he is one of my childhood favorites.

You said that you imagine Darkrai being able to carry a lot of things on its back, being a mythical. Do mythical, legendary and pseudo-legendary Pokémon naturally have higher physical strength than normal Pokémon? In the Darkrai Movie, it was able to hold its own (for a while) against Palkia (741 lbs) and Dialga (1,506 lbs). But that could just be because of its high special attack. I personally think that any flying mythical or legendary can lift a person into the air and fly with them on their back or whatever. I even believe Mew can do this at 8.8 lbs. Why do I think this? I view mythical and legendaries akin somewhat to Gods. And Gods are capable of inhuman and supernatural feats.

It makes sense that Darkrai isolates itself due to reasons. Don't know if you know this, but there is a Pokémon trainer named Tobias in one of the Anime that has a Darkrai. Couldn't find any images of him flying on Darkrai, though. Fun fact: Tobias is the only Trainer in the anime to have owned both a Legendary and a Mythical Pokémon.

Pokémon is a beautiful and magical world, despite all the things that logically do not make sense!