You have the chance to Mega Evolve your Pokémon in Pokémon Go. Only a select few can do it, and those that can require you to participate in Mega Raids to earn Mega Energy. The Mega Evolution Pokémon you battle gives you their Mega Candy at the end of it. For example, suppose you battle a Mega Blastoise. In that case, you earn Mega Blastoise Energy for defeating them, allow you to Mega Evolve your Blastoise. The transformation does not last forever.
Your Pokémon’s Mega Evolution goes away after four hours. It has a timer that you can view in the general menu where it shows its stats. When that timer hits zero, the Pokémon returns to its standard form, and you need to participate in other Mega Raids to earn more.
The timer forces you to choose the best times to Mega Evolve your Pokémon to avoid wasting your precious Mega Energy. When you first Mega Evolve your Pokémon, it costs quite a bit to do it the first time. For example, for Blastoise to Mega Evolve, it costs 200 Mega Blastoise Energy. It costs 50 Mega Blastoise Energy to evolve after that. The system keeps you involved to seek out Mega Raids continually, but it doesn’t make it a massive ordeal every time you want to use the feature.
More Mega Evolved Pokémon will release in the future, and there’s the potential to catch their shiny form after you win. Make sure to keep your eyes peeled for when they release, and expect to take on various Mega Pokémon in these raids. The faster you complete them, the more Mega Energy you earn by the end of the battle.