When one or more tamed horses in a world are dead, it rarely  (around once every 10 hours I play) happens that a player teleports to the back of one of the player's tamed dead horses, thus "reviving" the horse. Last time it happened around a week ago. It happens in my own world, regardless if I play alone or with my friends. It's not a realm and cheats are not enabled. Usually it takes you ~50 blocks and the location can be elsewhere than where the horse died. When I filmed this clip, one other player was online, but this also happens if I'm alone in the world.