GAMEFREAK adding psychic type onto Alolan Raichu is hinting at the Pikachu line being more closely related to Mew than other Pokémon. And the technique used to get Alolan Raichu in Pokemon Sun And Moon is evidence of that.
How to get Alolan Raichu in Pokemon Sun And Moon
In order to evolve Pikachu, a Thunder Stone is needed. The Thunder Stone is one of the most evolutionary items needed in Pokemon Sun and Moon. Upon reaching the second island, Akala, you will eventually head to Route 8 on your way to the lush jungle. Following the said route leads you to a small area that branches off to the right of the map. This area is right across the Pokemon Center. Trail towards the path and you will hit a trailer with a man searching for fossils. Head off to the right of the trailer, then take a step forward. Behind it, you will be able to pick up the Thunder Stone.
Alolan researchers don’t know why Raichu took on this form but there is a theory. Pikachu lived in a region of Alola where there are no naturally forming thunderstones. That being said, this unfortunate situation would not stop predators from hunting down Pikachus. Pikachu had to find a way to overcome the fact that it could not evolve and defend itself without the help of a trainer.
Pikachus are said to have eventually developed an advanced form of electrokinesis, the ability to create and control electrical currents with the mind. Tapping into other Mental abilities, they gained the ability to evolve into Raichu without the need of a thunderstone. There’s no concrete evidence to this theory except the fact that there aren’t any thunderstones in that area of the region but the people of Alola have come to admire this Pokemon because its potential back story is one of true survival and who doesn’t like a good underdog.