Vampire Cows are both vampires and cows, combined in a horrifying experiment by the Master. The Adjunct Professor has only encountered the Vampire Cows once, in the episode "Moo-rder".
Vampire Cows eat other cows and also people. They use their fangs to stab the victims, then drink their blood out of a bendy straw. They only come out at night and like to terrorize people in castles.