Now the official PR is here, we can confirm that a number of Generation III ghost-type Pokémon "originally discovered in the Hoenn region" are coming to the game, including Sableye and Banette.
Here's some more info:
Niantic, Inc. and The Pokémon Company International today announced an upcoming Halloween celebration in Pokémon GO with an in-game event that will herald the arrival of a number of Ghost-type Pokémon originally discovered in the Hoenn region, including Sableye, Banette, and others. From October 20 at 8:00 p.m. BST to November 2 at 9:00 p.m. BST, players will begin to see these mysterious Pokémon from the Hoenn region out in the wild, while also encountering an increased number of spooky Pokémon such as Gastly, Cubone, Misdreavus, Houndour, and more. We hope Trainers enjoy this special Halloween treat! Look forward to arrival of the rest of the Pokémon first seen in the *Pokémon Ruby* and *Pokémon Sapphire* video games, starting gradually as early as December.
For the duration of the Halloween event, special Pikachu will also be out and about, celebrating in costume and available for Trainers to catch. Candy rewards for catching, hatching, and transferring Pokémon will be doubled throughout the event, and your buddy will find Candy twice as fast. Trainers will also have the opportunity to stock up on special boxes from the in-game shop, featuring items such as Raid Passes and Super Incubators. Finally, players will be able to wear the all-new avatar item, Mimikyu’s Disguise Hat, created in the likeness of the mysterious Disguise Pokémon Mimikyu.
This will no doubt be exciting news for fans of Pokémon GO. Are you looking forward to Gen 3?