Pikachu was created by Mewtwo through cloning Ash's Pikachu's DNA. He started fighting with Ash's Pikachu the minute he saw him, although Ash's Pikachu didn't like it. This happened at another time again; however, the two Pikachus became friends.
In Mewtwo Strikes Back
Pikachu first appeared in the first Pokemon Movie, Mewtwo Strikes Back. After he was created, Pikachu fought with Ash's Pikachu. However, the battle came to a stop when Ash ran between Mewtwo's and Mew's attacks, which turned him to stone. Like all the other Pokemon, Pikachu felt utterly miserable about it.
However, the Pokemon's tears healed Ash. Mewtwo used his psychic abilities to lift Pikachu and the other clone Pokemon in the air, and they went away to find a safe home.
In Mewtwo Returns
Pikachu's next appearance is in Mewtwo Returns. The clones had found a lovely place to live, at Johto's largest mountain. Pikachu is seen with clone Meowth, talking to Mewtwo.
Later, after Team Rocket attempts to steal Ash's Pikachu, Pikachu comes to the place they crashed in. He uses Thunder Bolt to stop Ash's Pikachu's Thunder Bolt on Team Rocket.