Recently we updated our server to 1.6.1 and we went out, found two horses and a donkey. We bred the donkey and we ended up with with one donkey and two mules. First, my donkey disappeared. He did have saddle bags with stuff and he was named. Second, my friend's mule disappeared, she was riding it when it happened. It was saddled and had saddle bags with stuff in it. The last mule is still there but we think he may despawn soon. All three are tamed. We looked around the area they disappeared in and didn't find either.