Ditto is a pokemon from the original 150, and probably the most unique since in it's original form Ditto is practically useless. His special ability is to morph into the pokemon it faces. To normal players he's more a novelty, but more experienced players can use Ditto as a strategic (but risky) pokemon to take on tougher teams in the later parts of the games. I also remember Ditto having a pretty cute episode in the anime.
Ditto's Japanese name is メタモン(Metamon) which is based on the English words "metamorph" (Shape-shifting) and "monster".