Available since August 27, 2020 on Pokémon GO, the functionality of Mega-Evolution was already known to other games of the license since the generation of Kalos. The principle still changes a little since now you will have to collect Mega-Energy in order to Mega-Evolve your creatures.
To get them, you must participate in Mega Raids. Below is a list of currently available Raids, and Mega Evolutions that will arrive sooner or later in the game.
List of available Mega-Evolutions
List of non existing Mega-Evolutions
These Mega Evolutions are not yet available in the game but sooner or later will be.
