On my minecraft server, I was riding my tamed horse (with diamond armour). I disconnected, and when I reconnected, I was greeted with his twin brother. I tried this multiple times, and it still works. They're ridable still, so they're not just ghosts.
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.