It's more of a characteristic.ñ than a type. You can tell because a lot of flying pokemon gets the flying type, and if the pokemon needs another type they just get levitate instead. Or nothing at all. Exampled: Charizard X, Beedrill, Roaring Moon, etc.
But the biggest offender is Mega Altaria. A pokemon resembling clouds loses the flying type when it mega evolves. Why doesnt it lose the dragon type instead? When flying type is clearly it's main type based on the desing.