This bug has been in the game since the Skeleton Trap was added to a snapshot 15w38a. The bug is when a skeleton is riding the skeleton horse. (Skeleton Trap) and when the skeleton is trying to get you, he stops in his path to randomly jump and turn for a second. Then starts to chase you, then "dances" again. This counts for zombies, skeletons, and zombie pig men also riding a passive mob.