I know......
I used to have my horror dvds categorized by themselves at dvdaf.
Basically........you just make a new folder, label it "horror", then click all the movies that you want to add to that folder. It does have a limit though. If memory serves, you can only have 100 per sub-folder. I don't know.....been a while since I used my dvdaf account.
As stated earlier though..........guzzlefish does it all for you. For dvdaf, you have to go through your list and click the movies that you want to be added to the folders you have made and labeled for different genres.