Unlike evolving your Pokémon, which is permanent, Mega Evolution temporarily transforms your Pokémon, changing their appearance, improving their performance in battle through a significant boost in stats, and offering other unique benefits. You can tell if a Pokémon is eligible for Mega Evolution if you see a “Mega Evolve” button and “Mega Energy” on their summary page.
Mega Evolving a Pokémon may also change their type. For example, Mega Charizard X is a Fire and Dragon-type Pokémon, while Charizard is a Fire and Flying-type.
Note: Trainers can only have one Mega-Evolved Pokémon active at a time. Also, a Pokémon’s Fast and Charged Attacks don’t change when they’re Mega-Evolved.
The more often you Mega Evolve a specific Pokémon, the higher its Mega Level will be. Mega Levels are a special type of bonus that is only active while a Pokémon is Mega-Evolved. These bonuses can affect your gameplay in a variety of ways, including increasing XP catch bonuses for Pokémon of the same type as the Mega-Evolved Pokémon.

Viewing the Mega Pokedex

Tap the button in the bottom right of the Pokédex and select Mega to view all the Mega-Evolved Pokémon you’ve seen and caught.
Note: Mega-Evolved Pokémon are only marked as caught once you’ve Mega-Evolved them, not when you catch a Pokémon after a Mega Raid Battle. When you defeat a Mega Raid Boss, they turn back into their regular form before you encounter and catch them, so their Mega form only be marked as seen.