It is not known to evolve into or from any other Pokémon.
Ditto uses the move Transform to copy the opponent's moves, types, form, and non-HP stats. It can breed with any Pokémon other than the Pokémon in the Undiscovered group and itself, including male Pokémon and some genderless Pokémon, to produce Eggs of the other's species.
In its natural state, Ditto is a light purple or magenta free-form shape blob with vestigial facial features. It also appears to have two vestigial, handless 'arms' protruding from its body. However, it can take the form of anything, even inanimate objects.
Ditto is a genderless species.
Ditto is capable of transforming into an exact replica of any other Pokémon (an ability that is also shared with Mew), or indeed any physical object. However, if Ditto tries to transform into something based on memory, it may get some of the details wrong. Ditto will also be unable to remain in a transformed state if it starts laughing. Frequently, a Ditto appears that it can't transform its face. In rare cases it will lack the ability to increase or decrease in size, thus rather than creating exact duplicates, it will create (usually) miniature versions. Ditto cannot mimic the strength of another Pokémon. Ditto's ability to breed with any non-legendary Pokémon is also useful for creating Pokémon Eggs of rarer Pokémon that it may be difficult to get a mate for, such as the starter Pokémon.
Ditto's primary defense mechanism is its ability to rearrange its cellular structure. This permits it to transform into essentially any other physical object. When two Ditto meet in the wild, they will attempt to transform into each other. This tendency makes Ditto a unique Pokémon in battle, as it will adapt the form and abilities of its foe. Ditto is best able to transform into something if it has a model to work from. Although Ditto can transform via memory, it has a tendency to forget important details, particularly with its foe's face. Ditto will also transform into a rock when sleeping to avoid attack.
Ditto live in very diverse habitats, the most common being caves and forested areas. However, Ditto are never far from civilization or people. There have even been sightings of Ditto in places such as the Pokémon Mansion. They have been documented living in Kanto and Johto.
This Pokémon was unavailable prior to Generation I.