I came upon this bug when I was doing the quest Learning the Ropes (Learning the Ropes - Quest - World of Warcraft) All You have to do is stay on the horse when you turn it in and you can ride them anytime you want. (just make sure you don't hand in the next quest) You can ride them indoors and everything. Nothing big but kinda cool to use before you get your epic mount.
Also makes you immune to fall damage