New evidence has come to light that seems to indicate that a mainline Pokemon game was abandoned some time ago. The game was allegedly Pokemon Pink, a game that might have been paired with Pokemon Yellow, featuring Pikachu.
Pokemon Yellow released as a follow-up to Pokemon Red and Blue. Although the games bore strong similarities to each other, Yellow followed some of the plot points of the Pokemon anime, featuring an initially stubborn Pikachu and his new trainer on their journey from Pallet Town.
Not much is known about the alleged Pokemon Pink. According to @DrLavaYT, who specializes in 'Lost Pokemon History,' the canceled game Pokemon Pink is referenced in the source code for Pokemon Blue and Yellow, which recently leaked on 4Chan. What would have set Pokemon Pink apart from its fellow Pokemon games is unknown, though given it was referenced in Yellow's Pokemon code, it may have very well featured a unique companion Pokemon, like Jigglypuff or Clefairy.
Odds are that if the game's color referred to a singular Pokemon that it would have been Clefairy. Clefairy was originally planned to be the companion Pokemon for Ash in the anime, and even took on the role as starter Pokemon in the long-running Pokemon manga series. Though, both Jigglypuff and Clefairy require a stone to evolve, so from a gameplay perspective, there would have been similarities between Pokemon Yellow and a game featuring either one of the pink Pokemon.
If there was indeed a Pokemon featuring Clefairy in the works long ago, it unfortunately never saw the light of day, even as a remake. Pokemon Yellow was rebooted not too long ago as Pokemon Let's Go Pikachu for the Nintendo Switch, but Eevee was selected as the starter Pokemon for the game's twin, Let's Go Eevee. Why Eevee was chosen instead of Clefairy is unknown - if indeed Clefairy was ever even in the running - but what is known is that Clefairy hasn't had the same long-lasting overwhelming popularity that Pikachu has enjoyed.
Fans may never truly know everything that went on behind the scenes during Pokemon's development, but these scraps of data certainly pose an intriguing history.