From Bulbapedia, the community-driven Pokémon encyclopedia.
The following lists all the changes to Raid Bosses, in reverse chronological order, for Raid Battles in Pokémon GO.
Throwback Challenge Champion
May 29 - June 16
Throwback Challenge 2020: Sinnoh
Throwback Challenge 2020: Hoenn
Throwback Challenge 2020: Johto
Throwback Challenge 2020: Kanto
April 28 - May 1
Tricky Pokémon 2020
March 31 - April 28
Psychic Spectacular 2020
A Drive to Investigate
GO Battle League preparation
Team GO Rocket global take-over
March 2 - March 31
Pokémon Day 2020
Valentine's Day 2020
February 4 - February 25
Lunar New Year 2020
Level 1 to 4 Raid Bosses rotated for Lunar New Year 2020 from January 24 and reverted back on February 3.
Special Raid Weekend
For a Special Raid Weekend, Latias and Latios were added to the raid pool from January 24 to January 27.
January 7 - February 4
For Hatchathon 2020, Raticate and Wobbuffet wearing party hats were added to the current raid pool from January 2 to 16, 2020.
Pokémon GO Holidays 2019
Level 1 to 4 Raid Bosses were rotated for Pokémon GO Holidays on December 24 and reverted back on January 1.
Winter Weekend Event
For a special winter weekend event, Regice was added to the raid pool from 11 a.m. to 7 p.m. (local time) on December 28.
December 17, 2019 - January 7, 2020
Level 1 to 4 Raid Bosses were rotated for the Evolution Event on December 5 and reverted back on December 13.
November 26 - December 17
Level 1 to 4 Raid Bosses were rotated during Supereffective Week on November 19 to November 26, 2019.
Team GO Rocket Leader Disruption
Level 1 to 4 Raid Bosses were rotated during a Team GO Rocket Leaders disruption on November 7 and reverted back on November 14.
November 4 - 26
A Regi Return
All Raid Bosses were rotated for A Regi Return, in conjunction with the story event A Colossal Discovery, from November 1 to November 4, 2019.
All Raid Bosses were rotated for Halloween 2019 from October 17 to November 1, 2019.
September 23 - October 17
Makeup Legendary Raid Hour
On October 8, Mewtwo with the event-exclusive move Psystrike was available during a makeup Legendary Raid Hour.
Ultra Bonus 2019, Week 3: A Unova Unveiling
All Raid Bosses were rotated for Ultra Bonus 2019, Week 3: A Unova Unveiling on September 16 until September 23. Mewtwo caught during this time will know the event-exclusive move Psystrike.
Ultra Bonus 2019, Week 2: Global Challenge, Global Hatches
All Raid Bosses were rotated for Ultra Bonus 2019, Week 2: Global Challenge, Global Hatches on September 9 until September 16.