https://www.gamespot.com/articles/pokemon-go-legendary-pokemon-raikou-available-agai/1100-6460790/

A new month is just around the corner, which means a new batch of Field Research quests will soon be available in Pokemon Go. Developer Niantic has shared the first details about the upcoming set of challenges on Twitter, including which Pokemon awaits those who manage to complete enough of them. [Update: The new Field Research tasks are now live in Pokemon Go.]

According to Niantic, August's batch of Field Research tasks will revolve primarily around Electric-type Pokemon. As usual, the first task you complete each day will reward you with a stamp; if you collect seven of them, you'll achieve a Research Breakthrough and unlock additional rewards, including an encounter with a rare Pokemon.