My server is run with Craftbukkit #2815. I noticed that when a player that rides a horse, then disconnects while being on it and then connects again, the horse doesn't reappear.
I deactivated my plugins installed on my server and then reactivated all one-by-one. The one that seems to be the cause of the bug is multiworld.
I use the last version of MultiWorld (4.6.4)