Forgive my lack of using actual folklore. I'm bad at such research.
Everyone who has any level of study for the northern lands has heard of Fenris. Some say the legends are based on a creature called the Finnish Wolf, or Finn-wolf by some. In ancient days, before God revealed himself to the majority of man, a wolf was blessed and cursed. He could eat anything, and he would have to eat anything. This wolf may have spawned the legends of Fenris, or may have been created in an attempt to imitate the Great Wolf.
Finn-wolf is a creature who's hunger is insatiable. it devours every bit of flesh it can get its hands on. It can be fought off by the local tribes, but not killed. The ravenous hunger leads it to decimate wildlife in a wide swath before the beast needs to make a den (probably in some Regio) and sleep when its might is depleted. Its time awake can be shortened by damaging it (reducing its might) and keeping it from feeding on vis (which it needs to replenish its might). The Finn-Wolf was more frightening in its old days, when magical auras and magical animals were more plentiful, but there was a resurgence of tales when it found that it can feed quite happily on faerie creatures as well - which brought it into contact with humans once again.
Furthermore, finding the Finn-wolf is made more difficult by its infamy - as soon as it appears, its obviousness causes faerie versions of it to spawn from the stories that become the most vital topic in any civilization nearby. None of the stories can match up to the massive wolf beast that can swallow a small farmhouse to get to the sweet meat inside. Once Finn-wolf is sleeping, the Faeries that mimic it will last for years.
These familiars are not designed with too-specific wizardly goals in mind. Generally, they're of average power level, and inefficiently designed to make them feel more real and less game-y. In every one, I also had to ask myself, 'Why would someone choose this as a familiar?', in particular with any cunning-minded monster. In this case, pretend a Tytalus saw it as a challenge. Pretend a flambeaux just wanted to prove how good he was. Maybe you want a familiar who cannot die. Yes, yes, it's a high lab score to bind, but the harder part is finding a way to 'befriend' this unstoppable, unkillable monstrosity. The hard part is earning the right to bind it as a familiar, not the actual binding. It's a big magical brag, showing other wizards how big your magical staff is.
I suggest, if bound as a familiar, you get rid of the Ferocity for self-confident or Magical Sensitivity, or something fun like that. Lab Total 77 Perdo Animal. Yes, it's powers are Muto, but the beast just eats everything.
Death Prophesy on a monster is kind of a scary concept. You may also want to get rid of that - especially since the wolf is not bright enough to know what can kill it. Temporary might is also pretty miserable to keep track of, but it fits the story of the beast that wakes every few years.
Magical Might: 22Characteristics: Cun -3, Per +3, Pre +2, Com -2, Str +10, Stam +5, Dex +1 Qik -2Size: +4Season: WinterConfidence: 1Virtues and Flaws: Magical Animal; Death Prophesy, Ferocity, Great Stamina* x2, Improved Characteristics* x2, Puissant Brawl, Reserves of Strength, Sharp Ears, Tough; Essential Flaw(Ravenously hungry), Infamous, Magical Monster, Offensive to Magical CreaturesMagical Qualities: Gigantic x3, Improved Attack(bite) x4, Improved Damage (bite) x4, Improved Initiative x3, Improved Soak x2, Personal Power x4(Berserk, Jaws, Eat Aything), Minor virtue x4(Improved Characteristics x2, Great Stamina x2); Reduced Abilities x4, Monstrous Appearance, Temporary MightPersonality Traits: Dismissive of Small Things +2, Ravenously Hungry +6*, Wolf +3*Reputations: The Beast Who Devours All 4Abilities: Athletics (long distance running) 5, Awareness (Other hunters) 6, Brawl 14 +2, Hunt (big game) 7, Stealth 4, Survival (making a den) 5 [ 30 unspent ]Powers:Berserk Hunger(4 might): Finn cannot be stopped. His hunger over-rides all his senses, and he feels no pain or weakness. As long as he is not delivered a felling blow, he will fight unhindered by wounds. (ReAn 25;base 10, +2 sun, +1 constant, based on Endurance of the Berserkers; -1 might)Devouring Jaws(1 might): Finn-wolf has the urge to eat more and more. The massive beast can uhinge its jaws to massive proportions to swallow things that are almost its own size. This also gives it a +4 bite damage. (MuAn 20; base 4, +2 sun, +1 constant, +1 part; -1 might)Eat Anything(2 might): Finn is actually able to digest anything, though he finds non-meat to be tasteless. They all give him some level of nutrients, but only vis truly sates his hunger.(MuAn 35; base 15, +1 part, +2 sun,+1 constant; -2 might)Vis: 5 animal vis in heart or whatever you cut off.