Mega Evolutions were introduced in Pokemon GO on August 27, 2020.
In order to Mega Evolve a Pokemon in the game, a trainer will need to gather as much Mega Energy for that Pokemon as required. Mega Energy can be obtained by winning Mega Raids or completing Special Research and Field Research. If a Mega Raid is won in a short amount of time, more Mega Energy will be rewarded.
When a trainer is Mega Evolving a specific Pokemon for the first time, the cost of Mega Energy will be high. The price will be significantly lower every time that specific Pokemon is Mega Evolved after the initial Mega Evolution.
Once a Pokemon is Mega Evolved, it will stay in that form for a limited amount of time.
Niantic working on methods to improve how Mega Energy is earned in Pokemon GO
There are rumors that Niantic is currently working on more ways for players to earn Mega Energy in Pokemon GO. Some of these methods include walking with a buddy Pokemon and research tasks.
The developers are also said to be working on a feature that will allow trainers to earn bonus candy when catching a Pokemon that shares the same type as their Mega Evolved Pokemon.
Once a Pokemon is Mega Evolved, it can be used in Mega Raids, regular Raids, PVP Battles, and Team GO Rocket Battles. It can even be set as a buddy.
Mega Evolved Pokemon cannot be used in the Go Battle League or to defend a gym. When going into raids with a Mega Evolved Pokemon, all other Pokemon in the Raid will receive a boost in attacking strength. Pokemon that are the same type as the Mega Evolved Pokemon will also receive an additional boost.
Lastly, there are currently only 14 Mega Evolutions that have been introduced to Pokemon GO. This includes the starter Pokemon, Blastoise, Venesaur, and Charizard (who can evolve into two different Mega Evolutions). The latest Mega Evolved Pokemon to be introduced in the game is Mega Slowbro.