...So... this's something I've never seen before.
Way back during the making of the First Movie, Toshihiro Ono (creator of
The Electric Tale of Pikachu manga) was asked to make the origin of Mewtwo to coincide with the release of the movie. However, as time went on, Ono's manga was replaced midway by the short movie "The Birth of Mewtwo" instead. Ono's interpretation of Mewtwo's creation was released in an issue of CoroCoro Comic, and has been floating around in obscurity since.
Here's the manga, and
here's a synopsis - just to remember why Mewtwo's so awesome.