After riding on a horse, donkey or mule, they seem to disappear after you get off of them. I have not tried riding a pig yet to know if it happens with all riding animals. If you log out and log back in, the animal will reappear. For example, when me and my friend were playing on our server, he got off of his mule in the pen we created and said where did my mule go? So I came out to help find him find it and I could see the mule in the pen. I told him it was right in front of him and got on the mule. He said on his screen it looked as is if I were floating and he couldn't see the mule. He logged out and logged back in and could see his mule.

This same experience has happened to me several times while playing the game. Before I knew about the logging in and out fix, I had lost a horse in the desert. I've also been in a big mob fight and jump off the animal to only find it glitched on me again. It's quite a common bug that it making it highly annoying to ride animals in the pre-release.

I also want to note that this issue did not happen in pre-release 5. It was only when I updated the server to pre-release 6, that this started happening.