According to fan community The Silph Road, Pokemon Go update 0.79.2 (which was rolled out today by developer Niantic) includes a new loading screen featuring generation three Pokemon Dusclops, Shuppet, Banette, Duskull, and Sableye.
The first hint at generation two Pokemon in Pokemon Go came from a datamined update in 2016 more than a month ahead of the first generation two Pokemon appearing in the game.
A datamine of a new update has revealed that Pokemon from generation three are coming soon to Pokemon Go.
Last year, Niantic held a Pokemon Go Halloween event that offered double candy and increased spawns for Ghost-type Pokemon.