This page details prerelease information and/or media for Pokémon Yellow.
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 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.
These sprites were found at the very end of a list containing several sprites for an early version of Pokémon Gold and Silver, which itself was found within archives of code for that game. This isn't surprising, as Yellow was being developed around the same time as Gold and Silver. These sprites are labeled with the filename format "PMYF_" (most likely standing for "Pocket Monsters Yellow Front") followed by the corresponding Pokémon's Pokédex number.
74 of the original 151 Pokémon had their Japanese anime cries recorded and digitized, but none of them made it into the final game besides Pikachu. This is likely due to the very poor fidelity of the converted clips and the lack of space to fit them all in. Moreover, since many Pokémon have different names and/or voice actors between languages, this would also mean they'd have to be re-recorded in every language they wanted to release Yellow in (and then only if the anime aired in said language). Further compounding matters is that the anime Mewtwo doesn't have a cry, instead speaking in human language.
Small wonder the idea was scrapped!
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