I'd like to start out by saying I absolutely hate the mount system in this game. I hate it. It feels like a demo for a cool feature.
When mounts first came out I voiced this opinion (much less harshly) and provided a similar solution. I'd like to re-try. Let's address this as an entirely new feature and that is how I will describe it.
Each city has a stable, you pay the stablemaster some coin and he/she fetches you the horse you purchased. Different stables have different sizes, and thus different supply. It is possible to run out.
When you see the horse, it is a mob. It has all the qualities of a mob that acts like a horse. It does not yell if you attack it, because it's a horse. This mob has stamina and it loses stamina just as players do.
Anyone can mount this mob. (I sense the possibility of a new skill here) No one inherently owns this mob. It is a horse. When someone mounts this mob, it disappears into the long title of the character. What do I mean?
Billy Bob the warrior is here picking his nose. <--- Before mount
Billy Bob the warrior is here, mounted atop a white stallion. <--- After mount
When in this state, any hostile action towards the horse is directed instead toward the rider. AKA: You cannot kill the horse while a dude is sitting on it.
As Billy Bob rides around on his horse, the horse loses stamina. At some point, the horse will need to be dismounted and allowed to rest. Horses get tired, not Billy. Billy could ditch the horse and continue on foot if he is in a rush.
Some skills cannot be used while mounted.
Horses will respond to some commands:
Lead You move in a direction, taking the horse with you. It is just like moving, but the horse comes.
Hitch