Tibbles, the Annihilator
Humans as a group have caused the extinction of many species, but Tibbles the cat pretty much single-handedly took out the Stephens Island Wren. Owned by a lighthouse owner, the feisty cat had a great time hunting down every last flightless wren, who were helpless against such an effective predator. It is the only known case in which a single living animal wiped out a whole species. Evil: thy name is Tibbles.