Another suggested source for werewolves is that they are wolves who have managed to turn themselves into humans - most of the time. I first encountered this with Stan Freberg's Madison Avenue Werewolf, where a wolf transforms into an advertising executive. I next encountered this idea in Larry Niven's fantasy setting (it seems at first somewhat out of character, no? However, it is handled in proper Niven style).
Animals becoming humans is not a new idea. The pink dolphins of some rivers in South America are believed by superstitious locals to be able to become humans. One can find them at dances, only to disappear before dawn - sometimes after conceiving a child!
The werewolf's day/night dichotomy is related to Dr. Jekyll and Mr. Hyde more than, say, the day/night inversion of vampires. It indicates that there are two competing modes of thought: civilized or wild, thoughtful or thoughtless, morally aware or not. This interpretation of werewolves is standard enough that it has been used to indicate a state of psyche, even without a physical transformation. Hesse's Steppenwolf conceptualizes himself at first as a man-wolf, a shifting psyche half wild and half civilized.