What Are Mega Evolutions?
Mega Evolutions are temporary battle forms of various Pokémon. These Pokémon need to have a special hold item in order to activate known as a Mega Stone. When a Pokémon's Mega Stone is activated alongside a trainer's Key Stone, the Pokémon will Mega Evolve into a stronger form. This changes type and abilities of a Pokémon. In the games, it lasts for as long as Pokémon are in battle and you can oonly have 1 Mega Evolved Pokémon in your team at a time.
List of Mega Evolutions
What are Primal Reversions?
Primal Reversion is a phenomenon that works similar to Mega Evolution for two Legendary Pokémon. Like Mega evolution, the Pokémon react to a specific stone to change form. However, if they have the stone, they'll automatically change form.
List of Primal Reversions