make rouncey/palfrey/steppe/desert 6riding, destriers/coursers/arabians 7 riding, armoured horses 8riding.
add in "weight limits" for each horse so for example if you are riding a courser the total weight of your equipment can't exceed say.. 20? (this is just a random number but you get the idea)
make melee weapons do the same amount of damage to the whole horse as they currently do to the legs and nerf ranged damage to horses by atleast 1/2.
ez.