Disappearing Donkey/Mule while mounted

My friend was on her Donkey. The donkey was named and saddled, she was riding it across water. The night was approaching so she dismounted and the donkey disappeared. She could not hear it. But she logged out and logged back in, we also restarted the server and the donkey never reappeared, nor can you hear the donkey anywhere. The donkey also had look attached to it.