Yeah, if they want another "Uri Geller VS Kadabra" lawsuit. Those rings are trademarked.
But there's a bunch of animals that I'm really, really surprised Game Freak has never made into Pokemon. Dolphins. Houseflies. Earthworms. Dinosaurs. The amount of dinosaur Pokemon, I could count on both hands. How do they not have more dinosaurs!?
Hell, for a Japanese company that takes the odd bit of inspiration from Japanese mythology, it's amazing that how rarely they use the more well-known monsters. They've got, what? One tengu (Shiftry) and two kappa (Golduck and Ludicolo).
They've made a couple of oni, but it's not exactly obvious that they're meant to be one (kinda sorta Electabuzz, and Bert and Ernie... I mean Sawk and Throh). Give me a mean-looking ogre guy in a tiger-skin loincloth and a big club any day. (Plus, hey, excuse to use Bone Club/Rush/Merang on Pokemon that don't suck) Also, it could be another child-stealing Pokedex entry, as Japanese folklore sometimes has a child kidnapped and replaced with the child of an oni. Could be fun.