http://www.youtube.com/watch?v=CT3x9Og3fU4 here is a video of it.
I can understand the horse disappearing because I have a feeling that comes from optimization. When riding on a more of a normal terrain it's fine but the moment you start trying to climb rough terrain with the horse it just is glitchy.
Hell, I even managed to have this happen to me: http://cloud.steampowered.com/ugc/613842195068417191/EE40378FA6AF2BA2109F8A2A331D923DD804BBB6/
As a verdict, the game is amazing but would be nice if this got fixed.
