My brother started a single player world and opened the world to LAN. I joined, and tried to get on a horse that my brother had tamed. I was immediately cicked off (NullPointerException) and I could not get on the LAN server again; each time I tried resulted in the same error. In addition, each time I tried, the horse I was on was created in the server, essentially duplicating the horse.

What I expected to happen: Either I could ride my brother's horse the same as he could, without being kicked and without any duplicating horses glitch, or else I could not ride the horse because it is not my own.

UPDATE: this happens whenever you try to get on any horse; it doesn't have to be tamed, and it doesn't have to have any kind of saddle on it. It doesn't happen for the owner of the LAN server, only the people who try to get on it.

This is only tested on a LAN server, although I suspect it also happens on a multiplayer server.