Don't know if that is just a bad luck, however after the last update I lost 3 horses in one hour. I simply ride on it and all of a sudden disappear, both rider and the horse.
after re-loging there is no horse but I can still see the horse stamina, life bar and speed diamond, except I'm walking.
the only way to loose the horse indicators is to restart the game.
until now I found lots of free standing horses with an owner name on it and timer run out to zero, yet inaccessible.
it seams that Warhorses are especially varnable to this glitch, but that is just a guess.
oh, and suggestion,
at least 10 minutes horse timer.
it's really enjoying to worry about the horse timer especially if you contracting something.
Instead the horse should be available to others the instant a player is killed or knock unconscious.