I had 2 horses and a donkey: the horses with diamond and gold armour. The donkey had a full chest. After reopening the world, the donkey was gone. They all had been ridden. This happened on singleplayer.

Steps to reproduce:

1. Tame a donkey on SSP
2. Give the donkey a saddle and chest
3. Ride the donkey
4. Reopen the world
5. See how the donkey might be gone

This was very annoying as I lost a level 15 enchanted book.