Many of you may have noticed that the pet battle trainers - namely Varzok and Audrey Burnhep, along with many of the NPC pet battlers from Mists of Pandaria - are already in the game, but don't seem to do anything. You'll also notice that Varzok, Audrey, and the other racial-specific pet battle trainers (there's one for each major city, sometimes more in the cities that are home to more than one race) offer to teach you pets but don't actually have anything for you - yet.
It turns out that what they offer is a racial-specific pet! First, you have to train the ability to do pet battles - but you only have to do that once per Battle.net account. Each pet costs 50 silver (modified by faction discount; it cost 47s 50c for newly-made 85 premades on the beta) and is taught to you directly, just like learning a spell from a class trainer or a craft from a profession trainer, only here's the neat part - three of these are brand new pets. Pet collectors take notice! These pets are cross-faction, so if you can get 50 silver - and really, who can't get 50 silver? - it's worth your time to go cross-faction to pick up these pets. (They're also tradeable, so if you've got, say, multiple worgen, you can cage your extra ravens.)
To learn these pets, take 50 silver on a character who is of the appropriate faction and race to the pet trainers - Varzok works for the Horde, with one specific exception, and Audrey works for the Alliance - and learn it from them, at which point it will go directly into your pet journal.
These are pets that are new to Mists of Pandaria and are available in 5.0.4 from the Pet Battle Trainer after you train to be able to do Pet Battles. Some of them can be learned from other trainers - for instance, the Shore Crawler can also be learned from the goblin Pet Battle Trainer, Matty.
Goblin: Shore Crawler - Fun fact: this sucker is tiny. I guess the goblin only brought their tiniest crabs with them from Kezan.
Pandaren (either faction): Jade Crane Chick - Pandaren of both factions learn the same Jade Crane Chick. It's a pretty little thing!
These pets actually already exist, but if for some reason you don't have them and don't feel like getting them before Mists releases, you can wait until then and buy them for 50 silver. That's your call!
Draenei: Blue Moth
Night Elves: Great Horned Owl
Orcs and Trolls: Black Kingsnake
Blood Elves: Golden Dragonhawk Hatchling - Remember when I said there was an exception? This is it. It used to be that all of these pets were both race and trainer specific - for instance, only Matty would sell the Shore Crawler, and he would only sell it to goblins - but at some recent patch, all of these were made available on the 'central' pet trainers (Varzok and Aubrey) - except for the dragonhawk. Blood elves will find that Varzok has nothing for them, but Jarson Everlong in Falconwing Square in Eversong Woods will teach them this pet.
Tauren: Brown Prairie Dog