Regional Forms are alternate forms of Pokémon based upon the region they are caught or hatched. This changes the Pokémon's appearance. This allows for old Pokémon to be used a different way than expected
Alola Forms, otherwise known as Alolan Forms, are the Pokémon changed in Alola. They are all Pokémon whose species originated in Kanto. There are 18 different Alolan Form Pokémon.
List of Alolan Forms
Galarian Forms are the Pokémon changed in Galarian. These Pokémon introduce the first time there are Regional Forms of Legendary Pokémon and 6 of these Galarian Form Pokémon can evolve further, beyond what could be possible.
List of Galarian Forms
Hisuian Forms are Pokémon found in the Hisui Region, the region now known as Sinnoh. They are available within Pokémon Legends: Arceus. Even though Basculin isn't labelled as a Hisuian Basculin, it is considered a Hisuian Form.
List of Hisuian Forms
Paldean Forms are Pokémon found in the Hisui Region, the region now known as Sinnoh. They are available within Pokémon Scarlet & Violet.
List of Paldean Forms