My vote: if it's not an easy fix, remove mount stam entirely. Bluetails should be low hp and faster (as they are), Regular mounts should be medium hp (300 or 400) and medium speed, Battlehorns should be high hp and slightly slower than regular mounts. I think we've all learned that balancing can be a nightmare. While giving each racial mount different abilities, strengths/weaknesses would be cool, I don't know if the development time, as well as the time testing and balancing those mounts would be worth the effort. Just my 2 cents.