These are mobs that are spawned in Phase 1 of the Lich King encounter. I observed the following characteristics in 10 man (I have not done 25 man):
The first one spawns about 30s into the encounter, and another one spawns every 60s after that. The Lich King always raises them about 15 yards in front of himself (or, behind the tank who is tanking him).
They hit for ~8000-10000 damage on a generally ilvl 245 geared Death Knight tank.
They have two abilities, both of which are very dangerous.
The first is a Shockwave. It is a 1.25s cast, conal attack which does physical damage. It hit me for about 15k damage, and it can easily one-shot any non-tank class it hits. Note that unlike the warrior ability, it does not stun. As long as the Horrors are properly faced away from the raid, this ability is not much trouble.
The second is Enrage (I'm pretty sure this is the right spell). It is cast every 20-25s. It is a 1s cast which gives the Shambling Horror a buff that lasts for 6-7s. While the Shambling Horror has this buff, it hits very very hard - I took hits as high as 37k. Obviously, this extremely dangerous for a 10m tank.
According to the rogues in my raid, Shambling Horrors can be stunned.