Vampire Bat

Vampire Bats closely resemble more normal bats in general outline, but they are far larger, with wingspans of up to fifteen feet and bodies the size of large dogs. A Vampire Bat is further distinguished from a mere bat by its obviously Undead nature. Although its body is usually in far better condition than a Zombie or other lesser Undead, its pallid skin and red-glowing eyes are a clear warning to anyone who sees it.

Vampire Bats typically hunt alone, in which case they will seek out small groups of travellers or better still lone merchants and wanderers. Rarely, several Vampire Bats may hunt together, or be recruited into the army of a necromancer or Vampire Lord. In this case they will attack more openly, but otherwise with variants of their usual tactics—either overwhelming a group who have inferior numbers, or else carrying off victims.

Source

 * Warhammer Fantasy RPG 2nd ED -- Old World Bestiary (pg. 113).