Pokémon Pink
The leaked source code for Yellow includes a few mentions of a seemingly never made companion version that would've been called Pokémon Pink. This includes "Pocket Monsters Pink & Yellow", as well as pokemon_type in PM_DEBUG.DEF having the options "yellow package" and "pink package" (although the "pink package" has no effect on the game).
While it is not certain which Pokémon would have been the mascot of Pink, Clefairy appears to be the most likely candidate, given its coloration and the fact it was originally intended to be the franchise's mascot. An image of a frame that was included in the Japanese release of the Game Boy Camera appears to corroborate this.