The Pokémon Ditto, in its natural form, is a simple pink blob with a silly smiling face on it. Because of its copy ability, Ditto is one of the most important Pokémon in the series, but the most iconic aspect of Ditto's copy appearance outside the games all came from a single episode of the anime.
Ditto's gimmick in Pokémon has always been that it's capable of perfectly replicating an opponent, down to copying shininess in some games. However, a wide variety of merch and promotional material depicts ditto with incomplete transformations, where the Pokémon's usual face is replaced by the smiling Ditto face, revealing it as a copy. But if all Ditto are known for creating perfect copies, how did Ditto get to be known for these mistaken faces?
It all goes back to an episode of the original Kanto season of the anime, entitled "Ditto's Mysterious Mansion." In the episode, Ash, Misty, and Brock meet a trainer named Duplica, who is in possession of a Ditto. While the Ditto is capable of accurately copying a wide variety of Pokémon in size and shape, it's unable to alter its face, ruining the illusion. Duplica is putting on a show based around Ditto's copying abilities, so this is a serious problem for her. Ash and friends decide to help out, and work with Ditto to give it some practice, but to no avail. Team Rocket then kidnaps the Ditto and tries to get it to transform into rare Pokémon like Dratini, which it also fails to do (instead turning into the book). They actually end up threatening it into transforming properly, so the problem is solved when Duplica regains Ditto at the episode's end.
Duplica's Ditto Set the Standard
Duplica's Ditto with its imperfect transformations became the model for what Ditto could and should look like. Ditto disguised as Pokémon such as Bulbasaur in Pokémon Snap, for example, bear the original Ditto face, just like Duplica's. Other spinoff titles, like Pokémon Mystery Dungeon, have also used the imperfect Ditto transformation, to help distinguish a Ditto-copy from the original it's mimicking. It's also become common for plush toys of other Pokémon to come in a rare Ditto variant form, completely identical to the standard plush except for the Ditto face. Duplica would go on to appear one more time in the episode "Imitation Confrontation," where she has another Ditto which can copy appearances properly, but is unable to alter its size to match, creating to-scale versions of the Pokémon it's copying.
Given that Duplica hasn't appeared in an episode in over 20 years, many modern Pokémon fans may not even know that Ditto's silly smiling face copies all dated back to this first appearance of Ditto. It goes to show how influential episodes of Pokémon's anime can be when it comes to defining personalities for Pokémon, as these episodes linger in fans' memories for a long time.