Slimer

Slimer is a Hardmode enemy that only spawns in The Corruption and Underground Corruption. It starts off as a flying ball of slime, and when "killed" for first time it loses its wings and becomes a groundborne slimer. Slimers are actually two kinds of enemies put together, half bat, half slime, which counts as two sets of health.

Trivia

 * Its name may be a reference to the Ghostbusters films, which feature a flying slime creature named Slimer.