Swoop is the bat in Super Mario 64.
He appears in numerous places, such as Hazy Maze Cave. If you disturb him while he is sleeping upside down, he will fly towards you and try to attack you. He is very easy to dodge. However, if you disturb them in the toxic maze, you will probably be injured, as there are about 8 of them in that one area.
If you bop Swoop on the head, he will release a coin.