Vampire

Vampires are a Hardmode enemy that spawns only during a Solar Eclipse, during which they are as common as a Zombie. Vampires will turn into a bat if the player is above them, and attempt to fly near the player and turn back into a humanoid.

Trivia
The Vampire's 'Bat' form has a small face resembling Soma Cruz, the protagonist of the games "Casltevania: Aria of Sorrow" and "Casltevania: Dawn of Sorrow", though this may just be coincidence.