People who think J.K. Rowling invented the Phoenix.
I kid you not.
I have a great interest in myths and legends - like many people, I guess. More specifically, demonology and judo-christian lore (Key of Solomon, Paradise Lost, Divina Comedia, etc.). Of course, once you get in that territory, you come across some Aleister Crowley stuff as well.
And just in general, I don't have a problem when people use names of demons for their own character (Amon, Azazel, Asteroth are some of the more popular ones), just because "they sound cool". However, I do get annoyed when people learn about some mythical entity in a book and then assume the writer is responsible for its creation - even more so if they start altering the mythos surrounding it.
I'm not even sure if this pet peeve is against the writer or the reader.