I have 3 horses, bought 2 myself from stables and the other one was bought by my Steward, but when I use one and I unmount it the horse juts turn his back on me and keeps walking away from me like the stolen horses do, or sometimes it slides at a great speed away from me (looks funny but is annoying) as when I need it again I dont know where he went.
One last thing, this is not a bug but is something annoying too, it would be a good idea to make another npc get in charge of a store when the owner dies, Belethor died all of a sudden, the store is closed always and the guy who works for him keeps saying to go to Belethors shop to buy what I need, I went to the hall of the dead and theres Belethors coffin, so now I got a guy telling me to buy there but the store is closed forever. Adriane Avenicci was killed when some Vampires atacked Whiterun, bur her husband takes care of her store now, so something like that should apply to all the stores of the game, at least is what I think and I know some players would like that too.