Devolution (Japanese: 退化 degeneration) is the process of an evolved Pokémon reverting to its previous form. This is not nearly as well-known among the fandom as evolution is, due to devolution being impossible within the games and anime. In most media in which it exists, devolution is not nearly as commonplace as evolution, and is generally brought about mainly through artificial means, such as sprays or genetic engineering.
In the core series games
While devolution is not possible within the core series games, the notion is referred to by Slowbro's numerous Pokédex entries. Said entries claim that if the Shellder falls off, the Slowbro turns back into a Slowpoke. It is said that if Slowking loses its Shellder, it forgets everything it has learned.
In the spin-off games
Pokémon Mystery Dungeon
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Pokémon Trading Card Game
Pokémon cards can devolve in Pokémon Trading Card Game video games, which contain cards that can cause devolution.
The opening cutscene for Pokémon UNITE shows Talonflame, Slowbro, Greninja, Cinderace, and Garchomp devolving into their base forms of Fletchling, Slowpoke, Froakie, Scorbunny, and Gible, respectively. This suggests that experienced Pokémon that participate in Unite Battles are devolved to their base forms inside the stadium in order to evolve again during the game. However, Pokémon with baby forms, such as Pikachu and Mr. Mime, do not devolve to their baby stages.
In the anime
In the Pokémon anime, the subject of devolution has rarely been brought up. Misty explicitly states in Electric Shock Showdown that evolution cannot be reversed, effectively confirming the nonexistence of devolution within the anime. This was reconfirmed in A Stealthy Challenge!, where Bonnie asked Frogadier to devolve into Froakie so she could see it evolve again, but Clemont explained that it doesn't work that way.
In Dream a Little Dream from Me!, Ash's Pikachu had a nightmare featuring Frogadier having devolved into Froakie. Clemont's Bunnelby, which appeared as a Diggersby in the same nightmare, also devolved back into its first stage later in the dream.
In The Gates of Warp!, Pokémon in an alternate world version of Sinnoh devolved due to Dialga and Palkia's battle, with most of them eventually reverting all the way into Eggs. In Showdown at the Gates of Warp!, this battle was revealed to have been organized by Alternate World Team Rocket to make Pokémon easier to steal and train to serve them. Once they had been defeated, Ash, Dawn, Goh, Chloe, and their alternate world counterparts prayed for Arceus to stop the fight, which it did. Afterwards, Dialga and Palkia restored the reverted Pokémon to normal, fulfilling everyone's wish.
In the manga
Before Vee, Red's Eevee, evolved into Espeon, he had been the subject of an experiment by Team Rocket, which allowed him to evolve into any of his three evolutions that were known at the time. He was also able to devolve back into an Eevee and evolve into a different Eeveelution. Although this does mean that devolution is possible in Pokémon Adventures, the artificial means through which this was achieved and the lack of any other instances of devolution suggest that it may not be possible through natural means.
Akira's Rockruff has the unique ability to evolve into Lycanroc and then devolve into Rockruff again, thanks to the strange red stone on its neck. It also has the ability to change in between its Midday and Midnight Forms while being evolved.
Pokémon Pocket Monsters
Unlike any other media, Pokémon are capable of devolving just as easily as they evolve in Pokémon Pocket Monsters. This became especially evident when Red's Clefairy and Pikachu evolve into Clefable and Raichu respectively, then revert to their original forms, as well as when Green's Charmander evolved into Charizard and then turned back into Charmander.
In the TCG
In the TCG, devolution is perfectly possible, albeit not at a player's whim. Devolution must be induced by the effects of an attack, a Poké-Power, or a Trainer card. Examples include Mew's Devolution Beam attack, Flareon's Undevelop Poké-Power, and the Devolution Spray Item. The following is a list of cards that can achieve devolution.