Most bats are insectivores or fruit-eaters, but at least one species drinks blood.

Small, nocturnal flying mammal with natural sonar

Bats are nocturnal flying mammals. The statistics presented here describe small, insectivorous bats. They have a natural sonar that allows them to operate in total darkness; for game purposes, treat this ability as Darkvision. A group of normal-sized bats has no effective attack (at least in terms of doing damage) but can confuse those in the area, flying around apparently randomly. For every ten bats in the area, one creature can be confused; such a creature will suffer a penalty of -2 on all attack and saving throw rolls while the bats remain in the area.

Nocturnal, flying mammals that roost in caves or ruins.

Common bats that swarm when startled

This listing includes all types of bats. They will only attack when trapped, seeking to escape. When startled, they will swarm, causing confusion, extinguishing torches, etc. Because of their sonar navigation, their AC can rise to 4 given the right conditions, as they are nimble and swift.