Minecraft Username: SolaceDevotio Brief Description: I've had 4 horses disappear today. Instructions: I left them and when I came back they were just gone. I know, for sure, that the 4th one could not have been stolen. It was surrounded by a 5 block high wall in a town that only I have perms to. I did that because the other 3 had already disappeared and I thought they must have been stolen. How many times did you recreate this bug?: Twice. Once with 3 horses and once with 1.
People can attach the horse to a lead and fly away with it, make sure the horse is in a place without openings.
I'd suggest locking every single block of your stable with signs and trapdoors for floor and ceiling, that's what I did. 2 high block opnening and be sure there is no possible way a horse could be taken out, even if someone got in.