It was recently confirmed that there will be a Halloween-themed in-game event in Pokémon GO, but details about what the event will entail were intentionally left vague. However, after the announcement of the event, a rumor began to circulate about the inclusion of Generation 3 Pokémon into the game as part of the event. These rumors have been largely unsubstantiated until now.
According to Pokémon GO Hub, dataminers have looked into the game’s newest patch and found some interesting things. The most revealing of these is what appears to be a new loading screen. The game features a new loading screen every few months or so, and new loading screens tend to be based off of recent or upcoming holiday-based in-game events. Around Halloween last year, the loading screen was changed to one showing a Gengar hiding in the shadows near a trainer. In this recent datamining excavation, a new loading screen featuring Ghost-type Pokémon such as Gengar, Duskull, Dusclops, Shuppet, Banette and Sableye was found. Fans may notice that all of these Ghost-types, except Gengar, belong to the 3rd Generation of Pokémon, implying that these, and possibly more Generation 3 Pokémon, will be included in the game by Halloween.
Apparently, the cries for Generation 3 Pokémon can also now be found in the files, further implying that the new Pokémon will be joining the game soon. There will also reportedly be new Night Music coming to the game soon. As of now, there has been no word on what specific dates the Pokémon GO Halloween event will take place.