Disappearing Dogs

I copied my single player world to a server to play with some friends for a day. When I copied my world back to single player, my 20 dogs went that were tamed and in a wooden fence disappeared.

Not sure if this had to do with it but, I gave my friend some items that I wanted back after he left. I changed his username to mine temporarily in the "players" folder. I logged on to my server, got my stuff and logged out and changed the names back.

I don't know which of these things caused my dogs to disappear.

What I expected to happen:
The dogs would stay unchanged sitting in their pen and I'd continue happily.

What happened:
Alien abductions, apparently .