In the games
Since their introduction in Generation II, male Pokéfans have been accompanied with their children who are dressed up like Pikachu or Pichu. In Generation IV, these cosplaying children can also be found on their own, called Poké Kids.
The Colosseum Leaders in Pokémon Battle Revolution all cosplay as Pokémon. Taylor is dressed as Pachirisu, Marina is dressed as Kyogre, Rosie is dressed as Roserade, Voldon is dressed as Electivire, Terrell is dressed as Groudon, and Dusty is dressed as Lucario. All of these costumes can be Shiny and both forms are available for players as well.
In the anime
In the Dare da? for the Japanese version of The Bug Stops Here, Jessie turns out to be the subject, cosplaying as a Venomoth. When the "Pokémon" was revealed, the narrator says "Venomoth", but the children shout "Incorrect!"
Nicholai is well-known for his cosplaying. He wears the costume of any Pokémon he sends out into battle or of whatever Pokémon he might be trying to catch. He has been seen wearing costumes of Mudkip, Zigzagoon, Gligar, Tentacruel, Bellossom, and Zangoose.
Ash, Dawn, and Brock entered a Pokémon Cosplay Contest in All Dressed Up with Somewhere to Go. In this contest, however, it was their Pokémon which cosplayed, not themselves, though Rhonda, Officer Jenny, and Nurse Joy were respectively dressed up as Cherrim, Growlithe, and Chansey. Ash entered Pikachu, who did many impressions of other Pokémon. Dawn entered her Piplup as a Weedle, but lost when he broke character and used a BubbleBeam. The winner of the contest was Brock and his Croagunk, who dressed up as a Politoed and continued his impression of one throughout the entire episode.
In Climbing the Tower of Success!, a portion of the Bell of Wishes Festival contest required those who made it through to the round to dress up. The Trainer had to dress up as important Unova figure while their Pokémon dressed up as the Pokémon used by the figure. Ash, Iris, and Stephan dressed up as Alder, Professor Juniper, and Nurse Joy, respectively. Ash's Pikachu, Iris's Axew, and Stephan's Sawk in turn dressed up as Bouffalant, Darumaka, and Audino, respectively. Ricky was to dress up as Officer Jenny while his Elgyem was to dress up as Herdier but Elgyem used its Psychic powers to manipulate Miles's mind into thinking they were dressed up. Lastly, Cilan dressed up as Elesa and Pansage dressed up as Emolga. However, they did not pass to the next round because Cilan could not remember his lines.
Scene from YEAH!
James in a Moltres costume.
Meowth dressed as a Sunflora
Meowth dressed as an Aipom
Meowth dressed as a Banette
Meowth dressed as a Kirlia
Meowth dressed as a Murkrow
Meowth dressed as a Persian
Meowth dressed as a Shiftry
Meowth dressed as an Umbreon
Meowth dressed as a Wailord
Meowth dressed as a Weavile
In the manga
In the Pokémon RéBURST manga
While Cosplay doesn't officially appear in most Pokémon manga, there is a technique based off Pokémon Cosplay in the Pokémon RéBURST manga called Burst. Unlike usual Pokémon Cosplay, the Burst techniques are used for battle purposes.
In the fandom
Like most anime, manga and video game fandoms, cosplay is common within the Pokémon fandom. Most fans will dress up as human characters from the games and anime such as Ash, Team Rocket, female generational protagonists Misty, May, or Dawn, Officer Jenny or Nurse Joy. Many females crossplay as either Ash or James because of their youthful male looks, commonly known in Japan as bishounen, or "bishie" for short in the USA. Still, it is far from unheard of for fans to dress up in costumes of their favorite Pokémon species, especially if it is in the kigurumi or gijinka style of cosplay. Kigurumi is basically a hooded coverall pajama jumpsuit disguise of a character like Pikachu, while a Gijinka (also called "Moe anthropomorphism") is a human form of a species. Because of the popularity of Pikachu as the mascot of the Pokémon franchise, many young women dress in a Sweet Lolita style of dress at conventions.
Cosplayers who dress up like Pokémon are sometimes targeted as being furries, which have been the center of many controversies. However, while there is undoubtedly overlap, Pokémon cosplayers and furries are not one and the same; there are Pokémon cosplayers who are simply Pokémon fans and not interested in the furry fandom, and there are furries who would never want to wear a Pikachu costume.