The Kangaroo Monster is an extinct prehistoric species of kangaroo which was seen in the episode "The Transplant". Because they are such powerful beasts, only another kangaroo monster can defeat one.
According to the Computer:
"The Kangaroo Monster is a short-faced kangaroo, very rare, very extinct, very huge creature. A social outgoing creature that loved to crush anything that moved to a pulpy pancake. But that was then, this is now. They're all extinct and the bones are very, very valuable. Prehistoric Australia."