Eye-Bat is a small, red bat-like ghost with one big eye and long tail. It usually sleeps on the ceiling, then wakes up and attacks.
Eye-Bats first appear in the third level and generally appear in pairs or groups. In some cases, you can sneak up on them before they wake up and attack you; if you can manage to catch them still asleep, take advantage! They may take a couple extra hits but you won't have to continually dodge and aim, they'll just hang out without moving.