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 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. It is said that if Slowking loses its Shellder, it forgets everything it has learned.[1]
A similar situation applies to Exeggutor, as it is stated in its Pokédex entries that if one of its heads were to fall off, it would become an Exeggcute.
However, no Pokémon can devolve in any Pokémon game, with only two exceptions, both from Pokémon Mystery Dungeon series games. One of them is Pokémon Mystery Dungeon: Gates to Infinity, which introduced the Devolve Orb, a Wonder Orb that devolves all enemies in the room. Pokémon cards can devolve in Pokémon Trading Card Game video games which contain cards that can cause devolution.
Another example of devolution comes from Pokémon Super Mystery Dungeon, where the player and their partner evolve and devolve three times during the game's main story by the effect of Harmony Scraves they wear. This evolution is unlike regular evolution, as Pokémon with a middle evolutionary stage will skip right into their final form. It is explained in-game that the scarves were made from the Tree of Life, and their power activates when the scarves are brought close enough to the tree itself. This ability is lost when the scraves are sacrificed after clearing the Purifying Cave to bring the player's partner back.
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. 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 the manga
In the Pokémon Adventures 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 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.
In the Pokémon Pocket Monsters manga
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. Such instances are extremely uncommon. The following is a list of cards that can achieve devolution.
| Related cards Cards listed with a blue background are only legal to use in the current Expanded format. Cards listed with a green background are legal to use in both the current Standard and Expanded formats. | |||||||
|---|---|---|---|---|---|---|---|
| Card | Type | English Expansion |
Rarity | # | Japanese Expansion |
Rarity | # |
| Devolution Spray | I | Base Set | 
|
72/102 | Expansion Pack |
|
|
| Dragons Exalted | 
|
113/124 | Dragon Blast |
|
048/050 | ||
| Mew | 
|
Wizards Black Star Promos | 8 | ||||
| Wizards Black Star Promos | 9 | Mystery of the Fossils | 
|
||||
| Hyper Devolution Spray | T | Neo Discovery |
|
73/75 | Crossing the Ruins... |
|
|
| Pokémon Web |
|
031/048 | |||||
| Mew ex |
|
PLAY Promotional cards | 007/PLAY | ||||
| Mew ex |
|
EX Holon Phantoms | 
|
100/110 | Mew Quarter Deck | 003/015 | |
| Flareon | 
|
Rising Rivals |
|
19/111 | Bonds to the End of Time |
|
011/090 |
| Jolteon | 
|
Rising Rivals |
|
26/111 | Bonds to the End of Time |
|
028/090 |
| Vaporeon | 
|
Rising Rivals |
|
34/111 | Bonds to the End of Time |
|
014/090 |
| Jirachi |
|
Unleashed |
|
1/95 | Metagross Deck | 006/014 | |
| Call of Legends |
|
11/95 | |||||
| Golurk |
|
Dragons Exalted |
|
58/124 | Dragon Blade | 
|
023/050 |
| Boundaries Crossed | 
|
150/149 | Freeze Bolt | 
|
064/059 | ||