With the recent arrival of Mythical Pokémon Mew comes an increase in all first generation Pokémon in Pokémon GO. Those are Pokémon who appeared in the Kanto region, the setting of the original Pokémon Red and Blue games of the late 90s. The Kanto Celebration is going on now and ends April 17.
Initially the Gen 1 Kanto Pokémon were the only Pokémon available when Pokémon GO launched in Summer 2016. In the nearly two years since, Pokémon GO has added Pokémon from Gen 2 (Johto) and Gen 3 (Hoenn), which technically makes it more difficult to capture any specific Pokémon.
Certain rare Kanto Pokémon such as Snorlax and Aerodactyl will appear in Raid Battles during the event, allowing trainers to team up and capture them. Trainers can acquire special boxes containing Raid Passes to challenge these Pokémon.
Capturing any Kanto Pokémon during the event will reward double the Candy.
Recently Pokémon GO added an all new daily quest system called Field Reserach, which give short-term goals and rewards such as capturing a certain type of Pokémon or visiting PokéStops. More long-term goals were added as Special Research. The first works toward unlocking Mew, the original Mythical Pokémon from the Kanto region.
The Kanto region Celebration event ends April 17.