https://www.gamespot.com/articles/pokemon-go-legendary-available-through-research-qu/1100-6457931/

Last week's big Pokemon Go update introduced a new research quest system to the game, which gives players additional objectives to complete while playing. The system launched alongside a new storyline tied to the Mythical Pokemon Mew, but it appears that isn't the only Legendary players can discover by completing quests.

On Reddit, Pokemon Go players are reporting that the reward for their first "research breakthrough" is another encounter with the Legendary fire bird Moltres. Previously, players could only encounter Moltres in Raid Battles at Gyms. This required physically going to the location of a Raid and teaming up with other players for a chance to capture one, which is particularly a hassle for players who live in rural areas. Thanks to the new research quest system, players now have another avenue for getting Legendary Pokemon.