Pokémon GO is gearing up for its Spring Marathon event running May 12 to 18, bringing new debuts and themed bonuses to keep players moving. Pikachu appears in a marathon visor alongside its shiny variant, while Flittle and Espathra make their in-game debuts. The event also includes special bonuses and likely boosted spawns, giving trainers more incentives to get outside, rack up distance and take advantage of the limited-time content before it wraps.
Source: Official Pokémon GO blog and Pokemon.com