Shambling Horror

Main article: Lich King (tactics)

Not to be confused with Shambling Horror (Tirisfal Glades) or Shambling Horror (plague eruptor).

Shambling Horrors are vargul that spawns during the Lich King encounter in Icecrown Citadel.

Abilities

Enrage - Increases physical damage output by 200%. 1 sec cast, 7 sec cooldown. Is casted every 20-25s, and lasts for 6-7 seconds. (Heroic Only)
Frenzy - Increases the caster's attack speed by 50% and the Physical damage it deals by 100% for 10 min. It seems they will Frenzy when below 20% max health and pnly on HC mode.
Shockwave - Sends a wave of force in front of the caster, causing 150% weapon damage (physical damage) in a 20 yard cone. 1.25 sec cast, 3 sec cooldown. Unlike the warrior ability, it does not stun. It can easily one-shot any non-tank class it hits.

Note: This information is based on the 10 man version.

They will only spawn in phase 1.
The first one spawns about 30 seconds into the encounter, and another one spawns every 60 seconds after that.
The Lich King always raises them about 15 yards in front of himself (or, behind the tank who is tanking him).
As long as the Horrors are properly faced away from the raid, Shockwave is not much trouble.
Apparently, Shambling Horrors can be stunned. It should thus be stunned every time it casts Enrage.