Megatherium

The Megatherium (a name which meant "great beast) was a mammal native to Earth. It was a giant ground sloth, ancestor to the smaller sloths which existed alongside humanity. The Megatherium ate green leaves and could rear up a "good six metres" to reach those at the top of trees, though it was quadrupedal. (PROSE: The Doctor Who Dinosaur Book)

By the 21st century, they were officially extinct. However, Martha Jones once spotted one, earning herself 500 points in The I-Spyder Book of Earth Creatures. (PROSE: The Last Dodo)

The Fourth Doctor cited the Megatherium as one of his favourite animals from Earth's history. (PROSE: The Doctor Who Dinosaur Book)