Mew - Pokémon Origins

One of the most elusive Pokemon in the known world, Mew was spotted most recently in Pallet Town. 

Their DNA was used by scientists - headed by Dr Fuji - to create the ultimate Pokemon. The resulting clone, dubbed Mewtwo, would break free and flee to the Cerulean Cave after turning on their creators and destroying the lab. There they would remain until he was captured by Red.