A supposed leak of the source code for Nintendo’s Pokémon Blue and Pokémon Yellow suggests that the release of Pokémon Yellow wasn’t meant to be the only follow-up to the original two games, as lines referencing Pokémon Pink can also be seen in the code.
The original games, Pokémon Blue and Red, launched in the U.S. in late 1998 to critical acclaim. The development of Pikachu as the mascot of the franchise came along with the accompanying anime series of the game, where Ash Ketchum takes Pikachu as his starter, and following the success of both of the original games and the TV series, Pokémon Yellow launched a year later with Pikachu as the main cover star.
This also saw success and marked a significant step in the progress of the morale system with Pokémon that developed in the second generation of the game series. However, source code that was leaked from 4chan’s /vp for the two games implies that Pokémon Pink was intended to be released alongside Pokémon Yellow, with Clefairy the likely candidate for the cover star.
This appears to have been found after British hacker Zammis Clark had hacked into Nintendo’s servers in 2018, later for which he pleaded guilty of doing, and stole private data. Elements of the data taken have been leaked since, and so the legitimacy of the source code leak seems to be tight.
Clefairy was also one of the more praised designs in the original for much the same reason that Pikachu was, harboring immense cuteness that led it to being a minor mascot. However, with Pikachu proving to be a far more popular character, it’s likely that Nintendo felt that only one follow-up was required.