Continuity mistake: When Larry is taken home after being bitten by the werewolf, the door opens and Larry is seen staggering in. In the next shot, the door is seen opening again.
Continuity mistake: Early in the film, when the gypsy Bela becomes a wolf that attacks Jenny and bites Talbot, he is in true four-legged animal form, sans clothing. After being killed, his human corpse is wearing clothes although it is remarked that his feet are bare. Later, when Talbot transforms, he is a Wolf-Man, wearing clothes and walking on two legs.
Continuity mistake: When John presents Paul to Larry, he calls him "Captain Montford", but the credits shows him as "Colonel Montford". (00:00:35 - 00:03:12)
Continuity mistake: When Larry is talking to Gwen while Jenny is being told her future by the gypsy, he removes his hand from the tree, but the shot later he is still leaning against it. (00:52:02)
Continuity mistake: When Larry is going through his first transformation, he is seen wearing a white sleeveless shirt and light pants. When he's seen in the next scene as a werewolf on the prowl, he is now wearing a dark long sleeved shirt and dark pants.
Continuity mistake: When Montford goes to the cemetery and sees the gravekeeper's body, he takes the pipe that he is smoking out of his mouth. In the next shot when Montford's assistant shows up, he is seen removing the pipe from his mouth again.
Answer: Werewolves as a concept date back many years - this film originated many of the current conventions though, so the simple answer to your question is "the writers of this film". See the Wikipedia for more info - http://en.wikipedia.org/wiki/Werewolf.