I feel like I missed something here. When I look at a minion, there's a link to send it to the minion zoo. I clicked on the link just to see what would happen, and the minion disappeared. So, what/where exactly is this minion zoo?

It's a minion collection/battling system currently in beta testing for GA users. There's a thread on it here

Ohhh that explains it! I just activated my GA today. Thanks!