It only evolves to this form in the Alola region. According to researchers, its diet is one of the causes of this change.
– Pokemon Sun Pokédex
I like how Alolan Raichu rides on its tail like a surfboard (using psychic powers presumably).
I had an Alolan Raichu for my second Sun playthrough. It’s reliably fast, and hits pretty hard. I wish it could learn a Ghost move though, most Psychic Pokemon can learn Shadowball or something, but not Raichu. I also wish its ability stayed Static, it’s a lot more useful than Surge Surfer, at least for a main game playthrough.
Raichu has its own exclusive Z-Move (using Aloraichium Z, which has one of the best Z crystal names imo), which works off its Thunderbolt to become Stoked Sparksurfer, which has 175 base power, and guaranteed paralysis (if the target isn’t dead after the attack)