Cave bear

Cave bears are a type of large, prehistoric bear that died out about 10,000 years prior to the 20th century. Nimrod's Neanderthal tribe considered the cave bear to be holy and the tooth of one was thought to be a sign of great wisdom. Producing the tooth of a cave bear from his own pocket, the Seventh Doctor roused Nimrod from a state of unconscious. This also had the side effect on Nimrod of having him mentally relive the life he had led in his tribe. (DW: Ghost Light)