Revealing mistake: When Larry is attacked by the werewolf, Bela, there are two shots when they are behind a tree, when he is actually fighting a man. (00:15:40)
Revealing mistake: When the wolf man is caught in a trap an falls on a log, his shirt opens at the fist and you can see that he is wearing hairy gloves. (00:17:54)
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.