Hello everyone! This is nothing I've found by myself, but I thought it's a really cool thing so I'm going to share it here
So, if you want to screw around with pets that should not exist, here's the addon you need ("SekritPets"): http://www.mediafire.com/?yiaxqa80b1e2nwy
Use Instructions:
Don't bother unless you are on the PTR.
Firstly, do "/sek" to generate a list of all the "secret" pets you have in chat.
Next, you can use "/sek summon" (and then link the pet into the chat message) to summon the pet you linked.
You can use "/sek setX" (and then link the pet into the chat message) to set the linked pet to X slot in your active pet team (X can be 1-3).
Additionally, you can do "/sek tamer" to generate a list of all the "tamer" pets you have.
Have fun with that
Source: How to get Secret/Hidden Pets - Forums - World of Warcraft