Differences between the mounts?
According to the Mounts page of the DA wiki, the difference between the type of mounts is the number of melee hits your mount can take before throwing you off:
- Horses take one blow
- Harts take two blows
- Dracolisks take three blows
- Nuggalopes take four blows
Otherwise the differences seem to be aesthetic.
Some mounts have more "health" than others, allowing you to take more damage before being thrown off. Other than this, the differences are largely cosmetic.