The Mammoth was August Catcher's interpretation of a Wooly Mammoth. He stuffed it's head with some sort of book about swimming techniques as he always talked about swimming and breaststroke etc. It comes accross that he talks in a very dignified and gentleman maner.
Tom first meets the Mammoth when he wonders around the museum at night. The Mammoth helps Tom escape and hide a few times, and then at the end Don Gervase throws a cigarette at him making him catch on fire forcing him to run out of the museum and into the river. In present day slight scorch marks are visible.
It is indicated that when Don Gervase took over Dragonport someone from accross the river bought the mammoth along with other unique exhibits.