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 McCrimmon 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)

In 1860, Sir Edward Scrivener had the head of a hippopotamus mounted in his home, Gralstead House. (AUDIO: The Ghosts of Gralstead)

According to the Legend of Sutekh, the hippopotamus was among the animals associated with the titular Osiran god of evil and violence, Sutekh. Other animals associated by the Egyptian tradition with the cult of Sutekh included crocodiles, donkeys and pigs. (PROSE: Background)

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