Cave bear

Cave Bears are a type of large, prehistoric bears that died out 10,000 years ago. 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. Produding the tooth of a cave bear from his own pocket, the 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)