Ok, so, I'm going to do some GENERALIZATIONS about mythology and folklore here, and in my experience when you make a GENERALIZATION about mythology and folklore there is always some PEDANTIC SHITHEAD who tries to treat it as though you made a UNANIMOUSLY TRUE STATEMENT ABOUT THE ENTIRE BREADTH OF MYTHOLOGY AND FOLKLORE IN ALL CULTURES AT ONCE, and then tries to "Um, actually!" you with one obscure counter-example so they can get that sweet endorphin rush of feeling smarter than someone else on the internet. It is also one of my biggest pet peeves when someone does that, so I'm typing this paragraph to preemptively tell anyone who does that to this post that I am personally sending a demon from hell to eat your toenails tonight. As soon as you sleep he's going to slip his clammy fingers under your sheets, tenderly caress your feet, envelop one toe at a time with his grimy mouth, and slowly suck the nails off your toes. And they'll never grow back, either.
Ok, now to actually answer this question. The Wolf Man popularized what is currently the most popular take on werewolves - namely, that a werewolf is a person who involuntarily transforms into a wolf (either a normal wolf, a wolf/human hybrid, or a monstrous wolf beast) based on some sort of stimulus/trigger, such as the full moon or excessive emotional turmoil. By day they're a normal person, by night they're a ravenous beast.
While there are examples of this take on werewolves appearing in fiction before The Wolf Man came out (The Werewolf of London is a werewolf movie with the same rough premise that came out about a decade before), they were by no means the most popular/dominant take on the monster. There are, of course, dozens if not hundreds of variations on the werewolf concept in mythology and folklore (like that one that went memetic on here a while back, the Wulver, who turns into a wolf-headed man at night that gives people anonymous gifts of free fish), but GENERALLY SPEAKING, two were the most common: the Garwolf, and the Bisclavret.
(I am using these terms because when I first heard this distinction articulated at an academic conference, those are the two terms the speaker used, so don't come at me with the "Um, actually, that only refers to one specific story" bullshit or some other pedantic nitpicky criteria).
The Garwolf is the most common take on a malevolent werewolf in literature, and is explicitly a witch who transforms into a wolf in order to commit violence more easily. The witch does so by wearing a belt or cape of wolfskin, or by spreading a special salve on their body - either way, these transformation trinkets are generally provided by The Devil or some other evil spirit. The Garwolf is conscious of their actions while in wolf form, a murderer who commits their crimes in beast form both to hide their identity and to increase the gruesome spectacle of their kills.
The Bisclavret is the most common benign take on werewolves in literature, being a person who, for reasons that often aren't explained, has to transform into a wolf routinely. To do so, they take off their clothes, and they can't transform back into a human without first getting back into said clothes. The two main Bisclavret stories I know - Marie de France's "The Bisclavret" and the Arthurian tale of King Gorlagon - concern bisclavret werewolves whose wives discover their secrets, steal their clothes, and leave them trapped in wolf form. In both tales, the bisclavret is discovered by a king while hunting, and the king takes note that while the bisclavret may be a wolf, it acts more tame and intelligent than any trained hunting dog, and quickly adopts them as a loyal pet. In time the wife of the bisclavret goes to visit the king/noble at court, at which point the bisclavret acts predatory for the first time ever, snarling and biting at her. Realizing there must be some reason for it, the king/noble uncovers the truth, returns the clothes to the bisclavret, and punishes the unfaithful wife. What we take away from this is that a bisclavret werewolf is still human in mind while in wolf form, transforms reluctantly/against their will, and wishes to be purely human (or at least not lose humanity while in wolf form).
The Wolf Man essentially combines these two modes of folkloric/literary werewolves into one, giving us a human who is reluctant to transform because their wolf self is a vicious, dangerous monster. Like the Garwolf, they are a terror and a blight upon the world, but like the Bisclavret, their transformation is not by choice and they are trying to keep their humanity despite it. What The Wolf Man adds is the lack of conscious control in wolf form - there is no human intelligence behind the modern werewolf's actions, just a vicious, malevolent beast bent on killing as much as possible. It's a take that combines the dramatic elements inherent to the two main folkloric werewolves and heightens it by mixing them together with an extra new ingredient, and the result is incredibly compelling - I think there's a reason this mode of Werewolf story became the dominant one, to the point where werewolves who work differently than The Wolf Man feel like they're subversive and new even if they're explicitly in the older mode.
The Wolf Man is also the first really prominent story about a werewolf who infects others with werewolfism that I know of, but there's so much overlap between werewolf folklore and vampire folklore (and also witch folklore) that I'm not entirely sure there isn't a significant amount of folkloric precedent for that aspect.
