UPDATE: Currently the achievement to kill the Darkmoon Rabbit doesn't have any rewards tied to it, so how the pet is obtained is still a mystery! We might have to wait until the first week of next month to get a preview of the MoP Darkmoon Faire.
MMO-Champion did some datamining and our first NEW vanity pet in MoP is the Darkmoon Rabbit! It's not only new, it's of EPIC quality and so far it's Bind on Use. How this pet's obtained is unknown so far, but there's an achievement that requires players to "Slay the ferocious Darkmoon Rabbit." (That Rabbit's Dynamite!) I'm not entirely sure if it's related, but it might be considering the Darkmoon Rabbit (whatever it may be) is involved.
MMO-Champion also has some awesome screenshots of critters and mobs that I absolutely would LOVE to add to my collection (if they ever come in vanity pet form):
I'm going to head back in shortly and hopefully get some awesome screenshots and possibly video of some cool new pet stuff... if I can find it!