This has happened to me twice so far and I have had to kill my horses and tame new ones. This first time, I joined my world and all my horses were where they were when I left but they were frozen, i was not able to lead or ride them, when I tried to ride them you were able to move a little bit back and forward but no turning and could not go more than half a block. The second time, I was leading two if my tamed horses and they froze in the middle of when i was walking them. Their tail and head was able to move, but you were not able to move them and this time you were un able to mount them as well. As of the last time they froze, i have not gone back to my world so im not sure if they have unfroze yet.