Walruses were Earth marine mammals. They were among the animals kept in the sea room of Shaun Brett's monument to his deceased father in Alaska. When the Permians revived in 1994, they inadvertently used their bio-electric fields to fuse the walruses' bodies with those of the other animals to form monstrous hybrids. (AUDIO: The Land of the Dead)
On Hyspero, the Scarlet Empress sent a giant walrus to attack the Eighth Doctor's expedition. (PROSE: The Scarlet Empress)
The pieces of a 12th century Trondheim chess set were carved from walrus ivory. (PROSE: Dark Horizons)
While strapped into the Morok mind-reader on Xeros, the First Doctor was questioned by Lobos on where he had come from. He thought of a walruses and an image of some appeared on the device's screen. He identified them as old friends of his. (TV: The Space Museum)