A new "pet" gets added to every box on December 18th: When you get it, it has a blank image, but if you put it in your sig, it will switch its pet image every minute, and it will also link to the pet whose image it switched to, so you can't find it by clicking on it.
Every half-hour, it will move to a different group, and it will also move one of the pets in the group elsewhere, so you can't tell if the pet went to a group because the count went up.
It can't be sent to the Pound or deleted, but it can be traded. However, if it gets traded and it moves to another group before the other person accepts, the trade will get cancelled automatically. It can be found in the archives, but you can't wishlist it.
TL;DR: Chaos UR pet that doesn't even have its own image.
Every half-hour, it will move to a different group, and it will also move one of the pets in the group elsewhere, so you can't tell if the pet went to a group because the count went up.
It can't be sent to the Pound or deleted, but it can be traded. However, if it gets traded and it moves to another group before the other person accepts, the trade will get cancelled automatically. It can be found in the archives, but you can't wishlist it.
TL;DR: Chaos UR pet that doesn't even have its own image.