When riding horses, any player or any horse, chunks fail to load.

Once reaching the final 2 chunks that are closest to the unloaded part of the world, the player is suspended in air and cannot dismount nor move the horse. Only solution is to disconnect and reconnect, where the world will then load appropriately. (15w44b)

If the player is still riding the horse when they reach the edge, to load the chunks they need to dismount and the world will load again. (15w45a)

(Only tested in multi-player)