Lords and Ladies from the Lancre Witches arc features fae as they were in the old myths, as does the first Tiffany Aching book, The Wee Free men - both of which I highly recommend.

(hands over the mini-Rayquaza frostlass)
From the looks of it, Froslass's mythological inspiration is more like an ice-based will-o-the-wisp, drawing people off their paths and making them lost so that they die of hypothermia, and then dragging off their frozen corpses. Still creepy, but not the same thing. It might be close enough to prevent a Pokémon based on that particular fae, though, which makes me a bit disappointed.

What, one Pokémon based on an entire fae organization? How would they pick which member best represents the entire group? Still, though, Dark/Fairy would make an interesting type combination. We'll have to wait until the reveal of what Fairy-type's weaknesses and resistances to see if it's any good or not.

When I looked for another good example of a folklore fae, I found that a lot of them tended to be water-based or associated with water, like Ondine or the selkie. If the mythological inspiration follows, there are going to be a lot of Water/Fairy-types, which makes retconning Marill into that type combination even more confusing than it already was.
Oh, a good Fire/Fairy could be Nain Rouge, a gnome-like creature resembling a horned monkey that shoots fire out of its eyes and delights in arson and getting back at people. I can see a very effective move set from that concept. Plus, Pokémon's always needed a move that gives the user heat vision.

Wikipedia is a lot of fun, but now I've got three tabs open on Celtic mythology, one on the races of giants in Dungeons and Dragons, and another on the Elder Gods of the Cthulhu Mythos. Knowing me, one of them is eventually going to be about some obscure comic book character or an East European country with a name that's fun to say out loud. Well, back to my Wiki Walk.