Yay, necromancy, but I thought about this some more in light of the new mount system.
Beyond animal taming and into the realm of animal training, training mounts in things like speed would be most beneficial. I think extra speed could work as a temporary boost based on how well you're trained your mount and the limits of it's species.
Back to the original point, I do wish some of the creatures already in game were available as pets, although obviously ever creature isn't suitable for that role. The idea of having pets one can use to get eggs to sell, etc is a good one in my opionion.