NCF – Extended list of Neutral creatures.

If you create them based on the description, you might get a ranged unit that can fly. Of course, a ranged unit doesn't need to fly, but it could work as a mage.

A sad property of the game mechanics... Oh, I wish I could change everything from the ground up...
Regarding sieges: theoretically, it could be possible to make them fly. Like, jumping over walls on fiery steeds. But I assume there would be problems with the animation.