werewolves in the mythological sense, i refer you to Brysings comment.
there are people who claim to be otherkin, which i believe is an animal spirit or astral being inside a totally normal human body. only in the spiritual sense are they werewolves, not in the physically change under a full moon sense.
as for the origin, there are several theories, but the main reason for people to think half human half wolf is due to a condition known as hypotrichosis [see Jojo the dog-faced boy] in which extra hair grows all over the body in thick layers. in the milder terms its extra hair on the arms, legs and torso, but in more sever cases the face, hands and feet can be covered in a thick layer of hair making a person appear to be a werewolf.
and as always, no spell, bite, chant, potion, lotion, wolf skin, or any other method can change you into a physical werewolf. you are human, nothing will ever change this. you may pretend, even go in for surgery, but your genetic makeup will forever be human.