A Silithid Colossus
A Silithid Colossus is a huge creature with brutal strength and attack with great strength. In each Hive of Silithus sleeps one, awaiting battle.
After the Scarab Gong was rung and the ancient Gates of Ahn'Qiraj opened, the the horrors of the Silithid and their Qiraji masters were revealed. Massive Qiraji Resonating Crystals broke through the earthen crust of Kalimdor and the Colossus of the Hives in Silithus broke free of their cocoons beneath the Hives. They heard the call to battle and rose to serve their masters. They cast their massive shadow upon the whole of Silithus and marched into war.
It is possilble that more of those beasts are in Ahn'Qiraj.
From the Homepage of the Ascent Guild [1]:
Colossus of Regal
"The pictures don't really do them justice - they really are quite huge. The thing is, they're not really meant to be killed with any finesse, and the abilities reflect this (2500dd + huge knockback AE, that constantly kept killing our more fragile people... oh and it can do it twice in a row just in case some of your casters survive the first one). I mean technically you can position them quite nicely, but with the way it works (and how many people are in the zone), it's pretty much given that some random helpful person will train it as far away from that spot as possible (lets just say there's memblurs involved, which means that at a bad time a single warlock having a dot on it is almost certain to pull it to the warlock).
Oh and how does that make it zergable? Well, they don't regen hitpoints, and it's intentional. 2.9 million hitpoints and you have about 30 seconds before the AE of ownage comes your way. The best we managed was 19% in one run, which was a decent pace. Alas, we went to check out the Cenarion Hold fighting after two runs, and Fury managed to finish theirs (Colossus of Regal)."