Took the words out of my mouth.
First off, he's shaped like a Humanoid Pterosaur, he's basically the offspring if a Xenomorph mated with a Pterodactyl. Secondly, it's never really explained, so anything's possible. But someone had to design them, and it certainly wasn't Mother Brain who designed Ridley.
.......I think I found a plot for the next Metroid game
His shape doesn't confirm anything. Most call him an Alien Space Dragon out of convince.
Often, dragons have many different shapes. Some are like dinosaurs, like Ridley, Charizard, Dragonite, and many European dragons. While others are like snakes, like Shenron from Dragonball and many Eastern Dragons. Not sure if Bowser counts, maybe to some.
But often a few things are very common among dragons.
1. They are big, like Ridley
2. Most of them can fly, like Ridley
3. They are giant Reptiles, often the size and shape of some form of dinosuar. Like Ridley
4. Most of the shoot stuff out of their mouths. Usually, it's fire. Like Ridley.
So as far as I'm concerned he's a dragon. Until he's confirmed to be something else, that is.