Prove me wrong. You can't because what the fuck else would this be? Game Freak isn't even including Gen 2 evolutions of Gen 1 Pokemon in this game, so it only makes sense that the new Pokemon is one that's actually been there alongside the 151 since the very beginning.

Its dex entry is #000 and lists it as the Glitch Pokemon. It is a Ghost type, and has an in-game event in which it possesses the fossils in the Pewter Museum.