Developer: Game Freak
Development Range: 1992-1995, Game Boy
Pokémon Red and Blue is known for a rather secretive development cycle, with very few select materials being released to the public. While the Capsule Monsters pitch gave a simplistic idea on what the game would originally be like before settling into the games we all know to this day, the source code leak gave way towards dozens of previously-unseen elements, showing that development went through many different ideas than originally thought.
There's a lot of early sprites, tilesets, Pokémon designs and even early drafts of the Kanto region! Not only that, but there's data that gives evidence towards scrapped Pokémon that would end up as the notorious Missingno. in the final, which were the cause of balance changes that were made throughout development.