Bats spawn randomly in caves and also are often the first mobs to emerge (in a little pack of three) from an unplugged maw. Most dungeons also contain bats.
Like crows, bats spawn in two varieties, tend to travel in packs of three, and deal no direct damage other than raising your blood pressure because they're so obnoxious. They'll sometimes hang upside-down from the undersides of surfaces. They enjoy blocking your passage up or down a single-block column, and often gather to interfere in every possible way when you're trying to raid a dungeon.