Being caught in a sudden rain shower, our heroes are obliged to seek shelter. In the middle of a clearing, they find a small, ghastly cabin. Dragging the reluctant Misty, Ash and Brock enter the cabin, instantly getting the impression that they are not alone. Could this be a haunted cabin? They soon discover that it is the Copycat house in which the trainer of Ditto lives.
What if Ash's pikachu isn't a pikachu, but rather a Ditto? Let's look at the evidence. For this theory, I had to dig into the mythical land of Season One of the anime, when it was some bit good and not repetitive. So, Ash is a Klutz. This is seen several times throughout the anime, and began at episode one, as Ash forgot to change out of his pyjamas before heading to Professor Oaks lab. But all the pokemon were taken!
However, its face was different to other Pikachu, leading the others to believe that it was a new species. Ash doubted Ditto's ability in battle, but accepted Duplica's challenge. Ditto easily defeated Ash's Bulbasaur using Vine Whip. Team Rocket promptly decided to steal Ditto, coax it into the shape of a Dratini and present it to Giovanni as a gift.
Ash and friend are continuing their journey. It starts to rain and they start to run looking for shelter. They find a house which they think is deserted. Inside they see another Pikachu mirroring Ash's Pikachu. Misty picks up the new Pikachu and finds out there is something weird about its face. She drops it. Brock says that maybe this is a new type of Pikachu. Ash says that he wants to catch it.