In terms of do they exist that is rather more complicated. It depends really what you mean by a werewolf. The way I see it there are three definitions. 1. A human actually changes his physical form into that of a wolf. 2 A human takes on the spiritual form of a wolf. 3. A human believes he/she turns into a wolf.
So the first one – no, I don’t believe it is possible. I do not believe that a human can morph their physical shape into that of a wolf. That’s a personal belief, I do know one or two people who believe otherwise but rationally I can’t see any basis for there being any truth in it. I think some people may have been watching a few too many Robert Pattison films *twinkles*.
The second definition is far more likely. I do not follow a Shamanic path personally and nor am I party to the advanced practises of those who do. But Shamanism is very involved with working with animal spirits. I see no reason at all why a human could not assume the spiritual form of a wolf, either for astral projection or to take the characteristics of that animal into their workings. Like I say, its not my thing – but it may well be worth speaking to someone who practises Shamanism if it is something you are interested in. Some witches may use their physical animal familiar as a house for their spirit (now you know where JK Rowling’s horcruxes came from…) but again, not something I do. (Would you trust any part of your spirit to a degu??)
The third definition is the most documented. The medical term for somebody who believes they are a werewolf is “lycanthropy.” This is a medical condition often linked to schizophrenia. There is actual medical evidence to prove beyond a doubt that this condition exists but the symptoms only consist of a belief in transmorphing into a wolf (or other animal) not actually making the physical change.