Ditto

Ditto (メタモン Metamon) is a -type Pokémon. Ditto are unique as their only attack is Transform. They use this attack to copy the stats (except HP), moves (only 5 power points), types, and appearance of opponent. Another reason that they are unique is that Ditto can breed with any Pokémon in a day-care center, regardless of its gender.

In Pokémon Diamond, Ditto can be found in Route 218 between Canalave City and Jubilife City. It may be hard to encounter, so a little patience is needed.

To breed with other Pokémon, leave a Ditto together with the Pokémon you want to breed with in the day-care center in Solaceon Town. After a day or two, you can get an egg of the Pokémon you bred together with Ditto from the day-care man outside the center.

Ingame Power
In any 4th Gen. Pokemon games Ditto can learn new moves if a Ditto Transforms in to a Pokemon that has the move Rage and you catch it when it use Transform and its Hp. is in yellow or red it will have the moves of the pokemon in turned to and keeps them for ever.

And in any Pokemon 2nd Gen. to 4th Gen. Ditto can breed with almost any Pokemon

In the Anime
In episode 37: Ditto's Mysterious Mansion, Ash and his friends find a mansion that belongs to a girl named Duplica and her Ditto. This Ditto could not transform properly, but learned to by the end of the episode.