Mellan the Raichu came from a small village on the outskirts of a forest that are so thick and wild rumours have it that sometimes if you wander into the woods at night, you may walk right into the land of the Fae and become lost forever.
Mellan decided to wander into these woods one night and came back with a strange Mimikyu Hood. He refused to explain how he came by it as though he either can't remember or can't speak of it under some kind of spell but ever since he received his hood villagers report seeing a Mimikyu wander around the outskirts of the village making mischief and beconning pokemon to come play with them

Mellan's hood allows them to turn into a Mimikyu but that's not all it does. Sometimes he will not turn back into a Raichu but could become any of the pika-clones, and if he turns into a Plusle or Minum, then he will split in half and be both at the same time