I think it would be very hard to do, especially if horses aren't in the game( it is unconfirmed). But even if they are, they would have to be really hard to hit with a sword( seeing as how they would literally have to be clinging to the side of your horse), bows are to complicated to do while on a horse, and even spells would be hard. What happens if you hit your horse? It dies, falls on top of you, and you are stuck there, under a probably overfed horse, unable to attack, as you are eaten alive by some kind of wild creature, or cannibalistic bandit. But hey, if anyone can make it work, its Bethesda