Vampire Bat

A pesky little thing, the vampire bat spawns anywhere and everywhere with a light level low enough to spawn monsters...

Behavior
Vampire Bats will, out of preference, target players, followed by villagers, pigs, and other animals. They will flap randomly until they sense a target nearby (and they can sense from a long way away), at which point they will try to reach and bite the target, causing damage.

The good news is that, alone, they are simple to deal with -- one tap from a decent sword will do it. The bad news is that they bring friends. Lots of friends. A whole swarm of friends to bear down on their unlucky target.

Will burn in sunlight, so try to roost during the day in quiet, dark areas.

Loot
Vampire Bats will drop a Vampire Bat Tooth on death, plus a few XP orbs.