Vampire bats would be a larger version of the bat (added in the latest snapshot), except that they have red eyes, fangs, are black instead of brown, and they would be aggressive. If you wanted to you could go as far as them turning into vampires. Maybe you could be able to find vampires in coffins underground, which function like beds for vampires, and kill them for their fangs to be used in brewing new potions?
Just a short little suggestion I thought someone could improve on. A lot.