Blizzard tested Lord Marrowgar on the PTR 3.3 servers on Thursday, October 15th. This is the first boss in the Icecrown Citadel dungeon (10-players and 25-players versions available on normal or heroic-mode). It is a giant skeleton with a helmet. It's not clear if this is a placeholder, as he looks like some of the trash mobs in family-size.
When you zone-in, you will see Tirion Fordring and Overlord Saurfang (Horde) or Muradin Bronzebeard (Alliance). As soon as the event starts, Saurfang or Muradin will tell Tirion they will leave to get the Gunship above the Icecrown Citadel, and that he will await us when we get there.