Some Pokémon in Pokémon Go will now have the chance to become stronger past their final form and access their Mega Evolution. Traditionally in Pokémon, a Mega Evolution is only available in battle. But in Pokémon Go, you can Mega Evolve your Pokémon, and it can travel around with you. You can take it with you on raids or fight with it against Team Rocket. Will you be able to take it with you to use in the battle league?
When Mega Evolved Pokémon become available on Pokémon Go, they will not be prohibited for players to use in the battle league. Niantic has not said when they plan to introduce these powerful Pokémon to the mobile game’s PvP portion. Still, players don’t have to worry about fighting against a Mega Evolved Charizard X or Y anytime soon. It’s likely when you can use them in the battle league, they will have a battle league class exclusively available for trainers to use them.
For example, during the second season of the battle league Niantic created the Premier Cup. It comes out alongside the Master League, which allows trainers to use any Pokémon to have in their roster, regardless if its a Mythical Pokémon or its maximum CP stat. However, the Premier Cup follows these same rules, except for the fact about Mythical Pokémon. They are prohibited. A similar system will likely release for Mega Evolved Pokémon, allowing all trainers in Pokémon Go to participate in the battle league, regardless of the Pokémon they have available.
The Mega Evolved Pokémon is a concept that could introduce other alternative forms of Pokémon, such as Dynamaxing, which released for Pokémon Sword and Shield. We already have Galar Pokémon in the game, but even those Pokémon are available in small waves.