If you log out on a tamed horse on a server, the horse will remain but also a copy will be taken with you. When you log back in the horse will be duplicated.
Does not occur on single player
Doesn't matter if in creative or survival
Horse does seem to need to be tamed.
Duplicate of MC-13737, please use the search function to see if your bug has already been submitted. Currently over 55% of tickets are being closed as duplicate.