Vampires

A mysterious form taken by certain of the servants of Morgoth.

Tolkien's use of the term 'vampire' is never completely clear, but he does not seem to intend the sort of undead monster we typically associate with the word. Indeed, none of his characters actually were vampires, but at least two were known to take the form of a vampire on occasions: Thuringwethil and Sauron himself. From the context of their descriptions, these two seem to have transformed themselves into a monstrous bat-like creature so that they could travel swiftly, and it's this bat-form that's referred to as a 'vampire'.

Something akin to vampires also make a brief appearance in the account of the Battle of Five Armies, at the end of Bilbo's journey to the Lonely Mountain. There, a cloud of bats followed the Goblin armies. Those bats don't seem to have been actual vampires, but they attacked their foes in much the same way.