The Menagerie Achievement is an achievement awarded for discovering a pet Reward Room.
AI Description
You have discovered a pet reward room. From caterpillars who secrete vodka to basilisks who can turn mobs to stone, a good pet can make the difference between survival and the end of the road. Just remember what happened with Harambe. There ain't no zookeepers around to shoot the monster's ass if you bite off more than you can chew.
Reward: You have a key. It opens only one cage. Figure it out, Einstein.