I read a lot of American books and have seen this a lot of times as a defensive remark. But can anyone tell me where it originated from and exactly what someone means by it??
I'll hazard a guess and say it is from the days when horses were the predominant means of transport. Others that come to mind are... Get down off your high horse, Get off your horse and drink your milk (John Wayne)
I think it's just a way of making a statement more extreme, e.g. "F*** you, and the horse you rode in on!" would imply greater annoyance than simply, "F*** you!"