Algalon or Yogg +0 I should add. Since you don't need to slay the Heroic version of LK for the meta achievement to gain the epic mount, it sounds like he'll work much like Algalon or Yogg +0, which was not needed for the Ulduar meta achievement.
What this could mean is, Heroic LK may be the hardest encounter in World of Warcraft thus far. I'm calling it now, Muru, C'thun, 4H, Firefighter, Algalon and Yogg+0 may pale in comparison to Heroic LK.