I'm looking for the title of a book that's been bugging my bf for years - he read it in the early 2000s (he thinks that's when it came out) and when I asked him for a description, he said he thinks they were teens and all versions of classic monsters. One of them was Medusa in disguise hunting some kind of agents (and there are cutaway chapters where she's hunting them) and she maybe had braided hair? They may have either been ON an island or IN Ireland and were maybe collected together because they were told that Medusa was after them, but she was actually after these agents/government guys and there to protect them. Also, one of them was maybe a swamp monster?