I've done some reading on werewolf myths and stumbled upon the medieval tale of Bisclarvet and learned that lyncantrophy gets its name from King Lycaon.
And while Ars Magica has Bjoernar magi, skinchangers, shapechangers, shape changing faries and lycantrophes I've seen little of the modern depiction of a half-man half-wolf creature. The Mu An spells in the core book are all for the complete change to animal form, not the half-and-half shape as we all know them from the Werewolf RPG and Hollywood films.
Are there any examples of such half-way transformations in canon Ars Magica?
I assume it's well within the powers og Mu An so I'm kind of surprised that I haven't seen any.
I can't think of much that's half and half, but there are a few things that involve taking on a particular physical quality of a beast whilst otherwise in human form to gain an advantage:
There's the Theriomorphy mystery of House Bjornaer (page 27, HoH:MC), which enables you to take on a quality of your heartbeast by also undergoing an associated physical change (e.g. if you gain Keen Hearing, you also gain animal ears).
Eyes of the Cat, and similar spells, again involve taking on animal properties to get an advantage (in this case, Cat's Eyes to see in the dark).
In addition, Marcus of Criamon in Magic of Hermes is doing original research to make grafting organs on to animals to make a new creature possible. The recipient explicitly has to be an animal (it being a human is noted as requiring another breakthrough beyond the one he's working towards), but I think the "donated" organ might be able to be a human one.
I guess the Gruagachan magic also has something along those lines. The Give Shape guidelines give the recipient a monsterous figure when receiving powers associated with a shape such as the strengt of a bear.
That kind of self mutilation is not what I was looking for, but I'll take a look at it anyway cause Marcus might have developed some spells as part of his research that's not just about grafting bodyparts from one creature onto another.
One quick check later and Marcus has a few MuCo(An) spells that change a part of the body in order to get bonuses to certain activities. And off course one that gives the target a bulls head, complete with horns that can be used to attack with.
Erhm... no! Lycanthropy comes from the very common greek words lykos (wolf) + anthropos (man). A lycanthrope is then a wolf-man. King Lycaon, a mythical figure, just happens to have a name tied to his myth.
Yeah, the actual MuCo(An) low-level spells (such as Eyes of the Cat) already do this, albeit in a restricted way. As such, if you wanted to be somewhat anachronisitic, you could probably /5 spont a bunch of wolf-related abilities to get something kinda close to a wolf-man:
Legs of the Wolf (increased speed)
Ears/nose/teeth of the wolf (get the facial construct, plus hearing/tracking/bite)
Pelt of the wolf (cold resistance)
Claws of the wolf (unarmed attack bonus)
Increasing the general Strength/Stamina might be an issue, though - as a healthy human being, compared to a wolf, is actually pretty tough. You'd have to add in some other animal to get that, and at that point you'd maybe start to loose the general humanoid shape you're going for.
EDIT - if you wanted to do this as a single spell, you could probably start with the basic Eyes of the Cat variant, and then add in magnitudes for complexity. Or else go for a full-body wolf transformation, then +1 Part. (And maybe an additional +1 for Complexity, but I personally wouldn't bother.)
Faith & Flame has story seeds to do with werewolves - I don't have a PDF and am away from my hard copy so I can't give a page reference.
Hedge Magic - Folk witches have Shapeshifter as a power so could form shapeshifter cults. Gruagach - "Borrow the nose of the hound" (p68) transforms the nose, "Borrow the wings of the eagle" turns your arms to wings so the Gruagachan as written (and by extension the Trollsynir and Koldun variants) can do what you were thinking of.
The Lion and the Lily - The Children of Odin have a script for teaching minor magical focus (shape-shifting) so perhaps they would create strange shapeshifting spells.
The folk witches among the bouda (hyena shapeshifters) from Between Sand & Sea have access to the Theriomorph Mystery or Supernatural Virtue, which give them the ability to acquire Qualities or Virtues of a single animal.