the WoW Model Viewer by his friend, and apparently are in the PTR client in advance of Wrath of the Lich King. The names "Forgotten Crab" and "Icecrown Crab," we're told, were made up by his friend, but that font on the pictures looks familiar to me-- if you know where these pics came from, let us know and we'll credit them further.
At any rate, they look convincing enough to us. We know there are all kinds of new wildlife in Northrend (we saw a few of them at BlizzCon, including a killer whale-type of creature swimming in the waters off the Howling Fjord. So big crabs, icy or "forgotten," aren't too much of a surprise. Check the concept art gallery below to see what else we'll see up North.