Bats. Mammals of the order Chiroptera. Winged mice.
Bats encountered in the game have been corrupted into demonic slaves of infernal forces. As they are much aggressive and deadlier than regular vampire bats (some even suggestively originated from regular vampire bats prior to the corruption), take extra precautions to deal them.
They appear twice in the game, during the two East Wing boss fights. They'll basically fly around the battle, and occasionally swoop in and bite you while you're busy fighting the boss. A hit from an individual bat doesn't do much damage, but bats travel in swarms and several of them can hit you at the same time, adding up to lots of damage.
Additionally, a hit from a bat has a chance of knocking away your weapon, which can be a bother when you're otherwise occupied fighting a boss. A single hit from any weapon will kill a bat, but they're small and fast and very difficult to hit. You're best off ignoring them, keep moving to avoid their attacks, and try to kill the boss as quickly as possible so you won't have to deal with them anymore.