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.
Faster than a giga, strong as a rex, with some levels can get to 1k carry, can gather and eat both berries and meat,can even gather fiber, if you tame a high level one you have a super gathering and combat mount. Unmatched in utility.