This creature was created by applying the Immortals Handbook Teratoid template (which increases a creature by 3 size categories) and then the Fiendish template to a wolf.
Spawn of Fenrir CR 12
Chaotic evil Gargantuan Magical Beast (Augmented Animal, Extraplanar)
Init +4, Senses Listen +9; Spot +9; darkvision 60 ft., low-light vision, scent
AC 22, touch 6, flat-footed 22 (-4 size, +16 natural)
hp 200 (16d8+128 HD); DR 10/magic
Resist fire 10, cold 10; SR 21
Fort +18, Ref +12, Will +6
Spd 150 ft. (30 squares)
Melee Bite +25 melee (3d6+24)
Base Atk +12; Grp +40
Space 20 ft.; Reach 15 ft.
Atk Options smite good, trip
Abilities Str 43 (+16), Dex 10 (+0), Con 27 (+8), Int 3 (-4), Wis 12 (+1), Cha 6 (-2)
Feats Cleave, Improved Initiative, Iron Will, Lightning Reflexes, Power Attack, Track (B), Weapon Focus (bite)
Skills Survival +4
Environment Any evil-aligned plane
Organization Solitary, pair, or pack (7-16)
Treasure None
Advancement 17-24 HD (Gargantuan), 25-48 HD (Colossal)
The creature resembles a stocky, overmuscled wolf, but it is far larger than the largest elephant. Its jaws drip foul-smelling saliva; countless oversized, stained teeth jut outward from its hideous mouth. Its fur is striped crimson and blood-red.
The spawn of Fenrir are the descendants of the mighty wolf-fiend Fenrir, who is destined to eat gods. They are very small and weak compared to their monstrous sire, but are terrifying monsters to mere mortals. Only the mightiest heroes can oppose these creatures and hope to survive.
The average spawn of Fenrir is 20 feet high at the shoulders, 45 feet long (including a 10-foot tail), and weighs 25 tons.
Spawn of Fenrir have some spark of intelligence, but are still raging, speechless brutes. They have inherited a spirit of irrational, destructive violence.
COMBAT
A spawn of Fenrir has an enmity toward good outsiders and other obviously good beings, and will attack them in preference to other targets. Otherwise a spawn of Fenrir fights like a wolf (though with much greater power) except that it will attack even if not hungry, since it is driven by violent rage.
Smite Good (Su): Once per day, the spawn of Fenrir can make a normal melee attack to deal +16 damage against a good foe.
Trip (Ex): A spawn of Fenrir that hits with a bite attack can attempt to trip the opponent (+28 check modifier) as a free action, without making a touch attack or provoking an attack of opportunity. If the attempt fails, the opponent cannot react to trip the spawn of Fenrir.
Skills: A spawn of Fenrir gains a +4 racial bonus on Survival checks made to track by scent.