The Dire Bear is the most versatile mount in the game as they are small enough to fit in all caves. Their alt attack is also above that of a Rex, and with their sprint being comparable to a gallimus nothing will get away.
Bears make an excellent mount for exploring caves. If you can get about 3 high level ones you and other tribemates can explore through most caves with ease. And if you were to get in trouble these bears can run at high speeds so you can escape most predators.