https://www.gamespot.com/articles/pokemon-go-special-summer-pikachu-celebrates-games/1100-6460208/

Niantic's phenomenally popular mobile game Pokemon Go is celebrating its second anniversary this month, and the developer is commemorating the occasion in a number of ways. First, series mascot Pikachu and its pre-evolved form, Pichu, will appear much more frequently from July 6-31, and players will even have a chance to catch a special version of the Electric mouse for a limited time.

Throughout the remainder of July, a "summer style" Pikachu will also appear in the wild. Like the various hat-wearing Pikachu that were available during certain in-game events, this Pikachu only differs in appearance, sporting a straw hat and sunglasses. Additionally, players who have a gold Pikachu Fan medal will be able to purchase special items for their avatar, including a t-shirt and Pikachu ears.