Pokedex Entries, courtesy of Bulbapedia.
Red & Blue
Capable of copying an enemy's genetic code to instantly transform itself into a duplicate of the enemy.
Yellow
When it spots an enemy, its body transforms into an almost perfect copy of its opponent.
Stadium
Capable of reorganizing its cells to transform into an exact duplicate of its enemy. It is usually a shapeless blob.
Gold
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Silver
Its transformation ability is perfect. However, if it is made to laugh, it can't maintain its disguise.
Crystal
When it encounters another Ditto, it will move faster than normal to duplicate that opponent exactly.
Stadium 2
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Ruby & Sapphire
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.
Emerald
A Ditto rearranges its cell structure to transform itself. However, if it tries to change based on its memory, it will get details wrong.
Fire Red
It can freely recombine its own cellular structure to transform into other life-forms.
Leaf Green
Capable of copying an opponent's genetic code to instantly transform itself into a duplicate of the enemy.
Diamond Pearl
It has the ability to reconstitute its entire cellular structure to transform into whatever it sees.
Heart Gold
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Soul Silver
Its transformation ability is perfect. However, if made to laugh, it can't maintain its disguise.
Black & White
It has the ability to reconstitute its entire cellular structure to transform into whatever it sees.
Black & White 2
It can reconstitute its entire cellular structure to change into what it sees, but it returns to normal when it relaxes.
Capable of copying an enemy's genetic code to instantly transform itself into a duplicate of the enemy.
Yellow
When it spots an enemy, its body transforms into an almost perfect copy of its opponent.
Stadium
Capable of reorganizing its cells to transform into an exact duplicate of its enemy. It is usually a shapeless blob.
Gold
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Silver
Its transformation ability is perfect. However, if it is made to laugh, it can't maintain its disguise.
Crystal
When it encounters another Ditto, it will move faster than normal to duplicate that opponent exactly.
Stadium 2
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Ruby & Sapphire
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.
Emerald
A Ditto rearranges its cell structure to transform itself. However, if it tries to change based on its memory, it will get details wrong.
Fire Red
It can freely recombine its own cellular structure to transform into other life-forms.
Leaf Green
Capable of copying an opponent's genetic code to instantly transform itself into a duplicate of the enemy.
Diamond Pearl
It has the ability to reconstitute its entire cellular structure to transform into whatever it sees.
Heart Gold
It can transform into anything. When it sleeps, it changes into a stone to avoid being attacked.
Soul Silver
Its transformation ability is perfect. However, if made to laugh, it can't maintain its disguise.
Black & White
It has the ability to reconstitute its entire cellular structure to transform into whatever it sees.
Black & White 2
It can reconstitute its entire cellular structure to change into what it sees, but it returns to normal when it relaxes.