What it should do is enrage the horse so it charges back to where you are and headbutts the DK with a corresponding knockback, disarm and 60-second un-trinketable, un-dispellable stun. On the DK's screen, the scene should switch to a big INTERMISSION page with that dancing goblin from the "play free to level 20" ads all over the web until the stun wears off.
Meanwhile, you get back on your horse and a small cadre of nearby villagers walks up cheering. The mayor's beautiful daughter, single, 21 and heiress to a small castle-guarded island off the nearby coast, hands you a bouquet of flowers (with a handwritten note inviting you to meet her later that evening in a discreet moonlit glen) and you slowly guide your horse through the falling confetti and rose petals as the cheers echo from the village shops.