The WotLK EU Bestiary has been updated with a new creature: the Wendigo. Rumored to be related to the wendigo living in Dun Morogh, the wendigo of Northrend seem to lurk close to their caves. And, like other family-minded spawns of Northrend, these wendigo are fiercely protective of their relatives. The Bestiary warns of "the sound of their savage bellows," so I definitely hope to see some kind of sonic-based attack from the cool looking monster. The Bestiary also talks about the wendigo's bone-piles, so they might have a little more habitat in their spawning areas than we're used to seeing.
Like Alex mentioned previously, the Darkfallen are the hypothetical next-to-be-revealed monster. But there's only three unrevealed mobs on the total list, so at worst, the Darkfallen have a 33% chance to be the next spotlighted critter. I'm most curious about the Proto-Dragon, so my money says that's going to be the last one detailed.