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
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200 Mega Blastoise Energy needed
Water
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200 Charizard Mega-Energy needed
Fire and Dragon
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200 Charizard Mega-Energy needed
Fire and Flying types
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200 Venusaur Mega-Energy needed
Grass and Poison Types
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100 Beedrill Mega-Energy needed
Bug and Poison
List of non existing Mega-Evolutions
These Mega Evolutions are not yet available in the game but sooner or later will be.
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Normal and Flying types
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Psychic type
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Water and Psy Types
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Ghost and Poison Types
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Normal Type
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Types Bug and Flying
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Water and Dark Types
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Rock and Flying types
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Psychic and Fighting Types
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Psychic type
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Electric and Dragon types
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Steel and Ground Types
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Types Bug and Steel
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Bug and Fighting Types
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Dark and Fire
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Rock and Dark Types
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Grass and Dragon types
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Fire and Fighting Types
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Water and Ground Types
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Psychic and Fairy types
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Dark and Ghost
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Steel and Fairy types
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Steel
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Fighting and Psychic types
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Electric type
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Water and Dark Types
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Fire and Ground Types
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Dragon and Fairy types
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Ghost type
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Dark type
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Ice Type
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Dragon and Flying types
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Steel and Psychic types
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Dragon and Psychic types
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Dragon and Flying types
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Normal and Fighting types
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Dragon and Ground types
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Fighting and Steel Types
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Grass and Ice Types
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Psychic and Fighting Types
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Normal and Fairy types
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Rock and Fairy types