Devolution
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 the games
While devolution is not possible within the 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. Not only is this impossible to achieve in any game, but also, as stated above, no Pokémon can devolve in any Pokémon game.
In the anime
In the Pokémon anime, there has never been an instance of devolution at all. Moreover, Brock explicitly states in Electric Shock Showdown that evolution cannot be reversed, effectively confirming the nonexistance of devolution within the anime.
In the manga
In Pokémon Adventures
Before Vui, Red's Eevee, evolved into Espeon, it had been the subject of an expirament by Team Rocket, which allowed it to evolve into any of its three evolutions that were known at the time. It was also able to devolve back into an Eevee and evolve into a different Eeveelution. Although this does mean that devolution is possible in this series, 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.
In Pokémon Pocket Monsters
In the Pokémon Pocket Monsters manga, unlike any other media, Pokémon are capable of devolving just as easily as they evolve. This became especially evident when Isamu Akai's Clefairy and Pikachu evolve into Clefable and Raichu respectively, then revert to their original forms.
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 or a trainer card, examples being Mew's Devolution Beam attack, and the Devolution Spray trainer card. Such instances are extremely uncommon.
