Why are horses and donkeys considered different species even though they can interbreed

Horses and donkeys are considered as different species because the offspring which is produced due to the interbreeding of both the animal called as Mule.

This Mule is sterile and cannot produce offspring.

According the definition of the species, the organisms belonging to the same species should be able to interbreed and produce a viable offspring which can further reproduce among themselves.

fumikage isn't really a bird, and tsuyu does have teeth. tsuyu has a transformation quirk which still allows her to be partially human. otherwise we'd have a large frog running around. fumikage has teeth because again, he isn't a bird. his head was a quirk mixture if i'm correct.