Trying to consolidate Northrend pet information...
Albino Snapjaw, Hawksbill Snapjaw, Leatherbill Snapjaw, Loggerhead Snapjaw: No one really knows where these snapjaws come from, but a logical guess would be to assume they come from the Northrend fishing daily (probably have to catch a snapjaw for a daily fishing quest).
Baby Blizzard Bear: The Big Blizzard Bear came from Blizzcon and is a mount. No information on the baby.
Baby Shark: Sold for 60s in Lake Wintergrasp by Morgan Day.
Chilly the Penguin, Mr. Chilly: A reward for creating a Blizzard.com account, presumably only in the Beta, not in Northrend.
Cobra Hatchling, Proto-Drake Whelp, Tickbird Hatchling, White Tickbird Hatchling: Hatch from a Mysterious Egg which requires revered status with The Oracles. Each egg takes 7 days to hatch and can be any of the for pets above or Aged Yolk - basically a failure. Note, if you gain reputation with The Oracles, you lose reputation with the Frenzyheart. The Frenzyheart sell a Disgusting Jar at revered. So far no pets have been found in the jar, so I would probably go for Oracles reputation at the start of Wrath. However, the developers have stated that they want to make it easy to switch rep between the Oracles and Frenzyheart, so it most likely won't be an insane grind like switching from Scryer to Aldor.
Ghostly Skull: Skull that follows you around. Sold for 40g in Dalaran Sewers by Darahir.
Giant Sewer Rat: Caught by fishing in the Dalaran Sewers. Reportedly takes hundreds of casts.
Kirin Tor Familiar: Reward from Higher Learning achievement (sort of). To get access to Archmage Vargoth, you must have the Higher Learning achievement and it is presumed you will learn it from him or he will sell it to you.
Pengu: Requires exalt with The Kalu'ak. I would guess you either just buy it or it is a random reward from the Nurtured Egg.
Tiny Green Dragon, Tiny Red Dragon: No information yet.
White Tiger Cub: No information yet.
In addition to these pets, Wowhead also lists a lot of "Summon" spells that could turn out to be pets of some sort - no real information on any of these.
Summon Baby Murloc (Blue, Green, Orange, Pink, Purple), Summon Baby Pink Elekk, Summon Blue Racer (snake?), Summon Bronze Whelpling, Summon Cockatoo, Summon Cottaintail Rabbit, Summon Dart Frog, Summon Eagle Owl, Summon Elven Wisp, Summon Faeling, Summon Green Water Snake, Summon Island Frog, Summon Ribbon Snake, Summon Scarlet Snake, Summon Snowy Owl, Summon Spotted Rabbit, Summon White Plymouth Rock (??).
Hopefully I can keep this updated as I get more information. Most information was culled from WowHead, a little came from the great site, WarcraftPets.com.