Zeroken I really like your second example with having regional or racial differences between the horses. I would just be careful adding more abilities without increasing the point cost, which I am hoping would be avoided. How about the traits come with both a bonus to some stats and a penalty to others? For example a sturdier breed could have +1 save and -2 move or what do I know.
If they were not balanced this way, then I would only have these traits for the mounts for heroes. Though I do admit that the whole story is way cooler if it is applied across the board.
I would be afraid of the tome becoming too unbalanced for my mates to care to accept me using it, if the updates or additions going forward are not balanced.