Mega Starmie is now available in Pokémon GO after making its debut during Ultra Unlock: Starmie Super Mega Raid Day on Aug. 22. The Water- and Psychic-type Pokémon first appeared in Super Mega Raids during the event, with Shiny Starmie also available as a possible encounter. Starmie caught from Super Mega Raids unlocks Mega Level 1, giving Trainers who participated in the event a new Mega Evolution to add to their collection.
Source: Official Pokémon GO blog and Pokemon.com
Source:Pokémon
