Dragon Horse Glitch
Ive had multiple glitches while riding the horse, Shadowmere. Once, while fighting a dragon, the dragon did a dive bomb thing and Shadowmere was forced completely into the ground. My character's legs were partially in the ground too. As soon as I fast traveled, it was fine. Later, I dismounted while fighting a dragon and Shadowed charged. He ended up on top of the dragon, which then proceeded to fly away. I found shadowmere's body later on the road to Morthal (I was originally in Whiterun).