Well, you see,
buldings are a very rare and vicious animal. They're a cross between a bulldog and a starling. So it's basically a dog with wings.
However, being that starlings are so small, the wings aren't large enough to generate the lift required to get the bulldog portion off the ground!
So the bulding lives it's life in a constant state of frustration; having wings yet lacking the ability to fly. That's probably why Nathan hit it with his quad. It just couldn't fly out of the way in time to avoid a collision.
It also gets made fun of a lot by other animals.