Based on the comments and my very extensive knowledge of Pokémon, I can promptly say we don’t know.
It’s possible it’s a scrapped Gen 8 design, or possibly even Gen 9 design, but there’s no sources right now to clarify. As far as I (and others), can tell, this doesn’t belong to any particular “released” Pokémon.
I hope this makes sense! I’ll be sure to update if any new information about it comes out. In the meantime, I hope this helps.