Hippopotamus

A hippopotamus — commonly known as a hippo — was a large, Earth-native mammal that spent the bulk of its time in bodies of fresh water in jungles. The Second Doctor and Jamie once used a herd of animals — which included hippopotami — to stampede a group of Quarks. (COMIC: Jungle of Doom)

The Sixth Doctor once complained to a television station because, amongst other programming deficiencies, they allowed a blue hippo to be a newsreader. (AUDIO: The Ratings War)

Breeds
Pygmy hippos were a diminutive sub-species once encountered by K9 Mark I. (PROSE: K9 and the Missing Planet)