Late reply. Pichu is the pikachu varient from Gen 2.
Dont get confused with how Marill was "pikablu" in all the fanmade websites. People only called it "pika-blu" cause they didnt know what else to call it before Gold and silver officially came out.
Another thing you have to remember is that Marill evolves to Azurill. A RABBIT.
Pikachu is a mouse pokemon, not a rabbit.
Also Gamefreak and Ken Sugimori dont acknowlege it as a pika-clone cause it was never ment to mirror Pikachu.
A pikachu clone MUST BE ELECTRIC. If its not electric, its not a pika-clone.