What are Swarming Pokémon?
Swarming Pokémon events are the successor to the defunct Buneary Madness, which was last held in 2014. First introduced with Swarming Bunnelby for Easter 2017, the challenge is to capture as many swarming Pokémon as you can by the end of the time limit. The event durations have varied from 24 hours to 96 hours and events have been hosted for Easter, Halloween, Thanksgiving, and Shrimpy's birthday.
Due to the success of the first Swarming Pokémon event, Swarm Saturdays was also introduced as permanent reoccurring challenge on the RPG.
During a swarm event, escaped Pokémon randomly appear on pages of the RPG. If you spot one, you can click the image. A page appears with a captcha code. After entering the correct code, the Pokémon is logged as being caught by you. Rankings can be checked along with the number of Pokémon logged under the Swarming Pokémon tab in Challenges.
The Pokémon will look like this: 
Swarming Bunnelby was held a total of four times throughout both 2017 and 2018 in celebration of Easter. In 2017, the events were held over the course of two weekends (April 15-16 and April 22-23) with both events lasting 48 hours. In 2018, the events were instead held over the course of a single weekend (March 31 and April 1) with both events lasting just 24 hours.
Swarming Mimikyu occurred throughout the entirety of Halloween on October 31, 2018. Mimikyu became the first generation 7 Pokémon added to the RPG.
Swarming Pidgey happened on August 3, 2019, one day after Shrimpy's birthday. Unlike previous events which gave out a golden Pokémon to just the winner, Swarming Pidgey gave out a GoldenPidgey to everybody in the top 10, with the grand prize being a second GoldenPidgey.
Swarming Togepi and Swarming Exeggcute were held back-to-back for Easter 2020. Togepi's event ran from April 11th to April 12th while Exeggcute's occurred from April 13th to April 15, notably being the first three day long swarm event.
Swarming Horsea ran concurrently with Golden Day on August 2, 2020.
Swarming Ducklett ran for four days in celebration of Thanksgiving from November 25th to November 29th, 2020. In addition to rewarding the top 10 with GoldenDucklett, all participants who collected 20 or more Ducklett received a DarkDucklett.
Swarming Togedemaru was held three times alongside Golden Day on December 25, 2020, December 31, 2020, and January 1, 2021. Trainers who collected 25 or more Togedemaru on the 25th, 35+ on the 31st, and 45+ on the 1st received a GoldenTogedemaru, with the top 10 prizes also being GoldenTogedemaru.
Swarming Comfey was held on April 4, 2021 in celebration of Easter.
Top 10 Prizes
|Event||Grand Prize||Top 10 Prize|
|Swarming Bunnelby||GoldenBunnelby||ShinyBunnelby or DarkBunnelby|
|Swarming Ducklett||Collect 20 Ducklett||DarkDucklett|
|Swarming Togedemaru||Collect 25-45 Togedemaru||GoldenTogedemaru|
|Swarming Comfey||Collect 25 Comfey||ShinyComfey|
ToxiCity for the image.