After 16 months I finally got all of them. Shiny Costume Pikachu(s) : r/pokemongo
The Rarest Generation One Shiny Pokémon In Pokémon GO
Shiny Charmander | Etsy
Pokémon Go event Pokémon list: All costume Pokémon, including party hat Pokémon, flower crown Pokémon and every Pikachu costume explained • Eurogamer.net
Pokémon Go celebrates Halloween with Darkrai in raids and Pokémon in costumes - Polygon