It would be nice if when you unmounted your horse it still functions as a packhorse, following you arround and boosting your carry capacity. It might be nice to utilize the horse as a storage container who's weight disappears when you summon it. When you get off it stays and follows you and you can get back on by using the action key. However, when you are riding your whole carry weight and then some is added to the horse's burden. Like Avatars a burdened horse should move slower and lose the ability to run (or gallop.) It should also have a hard cap at which it won't let you add weight. To prevent bugged state, if you summon it when sitting on it would hit the hard cap you simply will summon it in pack horse mode.
An easier implementation might just be to have noncombat horse (mule, llama, etc) pets summoned as pack animals. They can increase your max weight carried by a set value, plus your mounted carry skill bonus. If they ever add a combat horse pet, that would be an exception. I don't think they operate as combat pets at the moment.