There are people who would use horses as a meat-shield. That is perfectly realistic since at medieveal times, there were such people. However there are other people (myself including) who would prefer to have a permanent horse at the start of the game, which you can train to greater speeds and strengths, and which you can cure should the horse (and since its a battle horse he WILL) become injured. So training should be an alternative, which would be cheaper but take longer time, while regular horses are still for sale to the first group of people :
I love that idea. Let's say you just finished setting up your character, making his face, etc., and then before you get to play a new menu pops up. You get to pick the color, name, size, etc. of your horse. The horse can not die, like a hero, however it can be knocked out & wounded. It can level up, like a hero, however you do not get to assign it's points. Depending on what it does most in battle changes the stats of it. So, if you just run around without charging anyone down, then your horse is more likely to gain more points in speed. If you make lots of tight turns in extremely hilly country to avoid falling off of cliffs, your maneuverabilty will raise. Of course, running over people will raise charging skill.
Your horse would also be able to wear armor that you purchase, but not unless it had a specific strength level determined by something like Strength - Weight > 6, then your horse can wear the armor. Of course, no one wants to ride a horse that only has a speed of 6.
Strength - Weight = Speed.
Weight + Speed = Charging Damage.
Speed - Weight = Maneuverability.
Or something like that could work for figuring the speed/maneuverability/charging damage of the horse with the armor.