Ferrumvorax, or, "An eight legged bear? I'll run it throug-AAARRRHHHHH!"

I really wish I could have found a pic for this one.

Ferrumvorax.

Frequency : Very rare
No. appearing : 1
Armor class : 0
Move : 10"
Hit dice : 12
% in lair : 40%
Treasure type : incidental
No. of attacks : 1
Damage/attack : 1-6/1-6/2-8
Special attack : knock down/pinon
Special defenses : Unharmed by blunt weapons, immune to poison, gas, normal fire.
Magic resistance : standard
Intelligence : low/animal
Alignment : Neutral
Size : M (6' long, 3' at the shoulder, 500 lbs)
Psionic ability : nil
Level/XP value : VIII/2800

The thought is put forward, by those learned in such matters, that perhaps the iron devouring Ferrumvorax is the result of that most fearsome beast, the Aurumvorax, an animal not native to this sphere, acclimating, or adjusting it's physiology to better suit life in a new world.

The Ferrumvorax is both larger, and more aggressive in attack than it's assumed progenitor. In fact, the creature is without fear, relentless, and fighting until killed.
It's coat is iron grey and streaked with rusty red. It's flesh, bone, teeth, and fur are bonded with the iron it eats.
A ferrumvorax will seek out and claim any large deposit of iron for it's own. Their appearance is especially dreaded in the mines of dwarves and gnomes.
A ferrumvorax which finds itself without a source of natural iron will develop the habit of preying upon travellers in order to satisfy it's need for the metal. As the creature requires flesh as well as iron to survive, the mailed fighting man is a most eagerly hunted prey. Those warriors most heavily armed and armored will always be a ferrumvorax's chosen target.
The ferrumvorax's initial attack will be to rear up on it's back four legs, and attempt to knock it's target to the ground. If the target creature weighs less than the ferrumvorax, a hit indicates that it has been struck off it's feet. If the hit was a natural twenty, the prey will be pinned beneath the ferrumvorax. The creature will use it's great weight to hold it's victim down while it savages it with tooth and claw.
A pinned victim must successfully roll to bend bars/lift gates in order to escape from beneath the ferrumvorax.
The hide of a ferrumvorax, in the hands of a master leather worker, can be made into a suit of leather armor which is equal to plate mail in protective value.