Devolution in Pokémon is something that might seem farfetched. It has never happened in the main Pokémon games. But the concept itself is possible in the Pokémon universe.
When you take a look at the entire Pokédex, a lot of them have evolutions that have specific requirements to evolve. So what does cause a Pokémon to devolve back into its previous stage?
There are different ways a Pokémon can devolve. However, in the main games such as Red and Blue, there is no possible way to do something like this. The concept does make an appearance in different forms of Pokémon media, such as the manga and video game spinoff Pokémon Mystery Dungeon. But in the form of a Pokémon from the Kanto region— Slowbro.
If it loses the Shellder that bit onto its tail as a Slowpoke, then Slowbro devolves back into the dopey Pokémon we all know and love. It potentially loses its memories of when it was Slowbro in the process.
We could also use Mega Evolution as a way of looking at what Pokémon “devolve.” When a Pokémon Mega Evolves, it returns to its previous form after the battle. Of course, this isn’t devolution. It’s how Mega Evolution works, and is more of a temporary evolutionary state.
So yes, when it comes down to it, Devolution is not a thing in the Pokémon series officially. For the side games, it is something merely mentioned, though never sees the light of day.