The regional variants introduced in Pokémon Sun and Moon are called Alolan Forms.
These forms represent Pokémon adapted to the different environments and microclimates of the Alola region. They can have altered appearances, types, abilities, and evolutionary relationships compared with their familiar counterparts.
Examples include Alolan Vulpix, which becomes Ice-type, and Alolan Raichu, which gains the Electric/Psychic typing. Other first-generation Pokémon with Alolan Forms include Rattata, Sandshrew, Meowth, Geodude, Grimer, and Exeggutor.
Alolan Forms are not the same as Mega Evolutions. A regional form is a persistent variant associated with a particular environment, while Mega Evolution is a temporary battle transformation available to certain Pokémon under specific conditions.