From Bulbapedia, the community-driven Pokémon encyclopedia.
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 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. A similar situation applies to Exeggutor, as it is stated in its Pokédex entries that if one of its heads fall off it becomes an Exeggcute. However, no Pokémon can devolve in any Pokémon game, with the exception of TCG video games, which contain cards that can cause devolution.
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 nonexistence of devolution within the anime.
In the manga
Before Vee, Red's Eevee, evolved into Espeon, it had been the subject of an experiment 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 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 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 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. Such instances are extremely uncommon. The following is a list of cards that can achieve devolution.