Sabre-toothed tiger (An Unearthly Child)

A sabre-toothed tiger (PROSE: Doctor Who and an Unearthly Child, The Eight Doctors) lived on Earth, circa 100,000 BC. (PROSE: The Eight Doctors) It had yellow fur. (PROSE: Doctor Who and an Unearthly Child)

On one day, it tracked the First Doctor, Susan, Barbara Wright, and Ian Chesterton through a forest, (TV: An Unearthly Child) but hesitated to kill them due to their strange manner and attire. (PROSE: Doctor Who and an Unearthly Child) When Za and Hur arrived on the scene, having been also tracking the TARDIS team, it attacked Za. Both were injured in the process, (TV: An Unearthly Child) with the sabre-toothed tiger being left with Za's spearhead imbedded in its side as it fell upon Za. It then retreated into the forest, (PROSE: Doctor Who and an Unearthly Child) whereupon it ran past the Eighth Doctor, who surmised that it would not survive the injury. (PROSE: The Eight Doctors)

Behind the scenes
The sabre-toothed tiger was never seen or named in An Unearthly Child, however it properly appears in both the novelisation and The Eight Doctors, which both confirm its identity as a sabre-tooth.