How to Mega Evolve Pokemon in Pokemon Go is an altogether different process than it was in the main series games in the franchise. In previous games Pokemon could be Mega evolved through the use of Mega and Key Stones found by or given to the player throughout the course of their adventure. The Mega Stone was given to the Pokemon as a held item and the Key Stone was a Key Item represented in-game by a bracelet worn by the player character. In Pokemon GO, however, the way Mega Evolution Works is completely different.
How to Mega Evolve Pokemon in Pokemon GO
In Pokemon Go, Mega Evolution is handled in much the same way evolution is, through the collection of a species-specific consumable item. To Mega Evolve a Pokemon, players first have to battle mega evolved Pokemon in raids to collect Mega Energy. This Mega Energy is species-specific so for example, defeating Mega Beedrill will yield Mega Beedrill energy.
Once enough Mega energy is collected, a Pokemon can be mega evolved for a period of time until it runs out of Mega Energy. The amount of energy needed to Mega Evolve also varies from Pokemon to Pokemon so for example, Beedrill only needs 100 Mega Beedrill Energy to Mega Evolve whereas Blastoise needs 200 Mega Blastoise Energy. Mega Evolve Pokemon can fight in raid battles and gym battles, however, they cannot be left to defend gyms.