I'm still waiting for our boys to meet a Black Dog. the first couple seasons they always mentioned them as a possible lead, but they never faced one.

Besides that I think it'd be fun for them to meet a Banshee, Pooka, or even better yet a Selkie. Hot girl with a hair fetish, a mischievous shapeshifter, and a shapeshifting dude who knocks up girls for fun. Things could get interesting.