There are places in america and other parts of the world where herds of wild horses live, well-adapted to the environment. if mules are so much better adapted to harsh environments than either horses or donkeys, why have there never been populations of wild mules?

simple mules are a mix of hourse and donky and are born infertle they cant BREED.

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.

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.