"Ditto rearranges its cell structure to transform itself into other shapes. However, if it tries to transform itself into something by relying on its memory, this Pokémon manages to get details wrong."
Ditto is one of the true anomalies in the Pokemon world, because it's really hard to define what it truly is. It's the only Pokemon in the world (all seven generations) that can transform into other Pokemon, and it's hard to understand why that is. Despite that, Ditto has a large popularity among gamers. Not necessarily for its battle skills, but rather, how it helps in Pokemon breeding. In the anime, there was even a Ditto trainer.
Ditto's Basic Attack is Transform, while its Special Attack is Struggle.