Just in time for trick or treating, Pokemon Go has a special event going on for the rest of October.
Here’s the official announcement (emphasis added by me):
Gastly, Drowzee, Cubone, and other spooky Pokémon are back again this Halloween, and this time, they’re bringing some special friends! Sableye, Banette, and a few other Ghost-type Pokémon originally discovered in the Hoenn region will be making their first-ever appearance in Pokémon GO. The rest of the Pokémon first seen in the Pokémon Ruby and Pokémon Sapphire video games will gradually arrive starting as early as December! We hope Trainers enjoy this special Halloween treat. The Halloween celebration will start on October 20 at 12:00 P.M. PDT and run until November 2 at 1:00 P.M. PDT.
A special Pikachu will be out celebrating Halloween in an all-new costume, and you can, too! Dress up your avatar for the occasion by wearing Mimikyu’s Disguise Hat. You’ll also earn extra Candy while you’re out trick-or-treating with your Buddy Pokémon: Candy rewards from catching, hatching, and transferring Pokémon will be doubled, and your buddy will find Candy twice as fast during the Halloween celebration! Last but not least, you’ll have the opportunity to stock up for your upcoming adventures with special boxes from the in-game shop, featuring items such as Raid Passes and Super Incubators.
While you’re out looking for Sableye, Banette, and other spooky Pokémon, use your Pokémon GO AR camera to capture festive photos and share them on social media using #PokemonGOHalloween. As always, make sure to remain alert and aware of your surroundings while playing Pokémon GO this season.
—The Pokémon GO team