Pokémon Gold and Silver were once completely different games. Nintendo debuted a playable demo of the second generation games at Space World in 1997, but by the time the games launched two years later they had a new map, re-designed Pokémon (as well as dozens of cut Pokémon) and significant story changes.Fans have spent decades trying to track down solid details about this early build with limited success.
That all changed today as an anonymous user has leaked a ROM file of the prototype online. The talented folks at The Cutting Room Floor have been feverishly digging through the file to unearth as many secrets as possible ever since, and quite a lot of major changes have surfaced already. This includes tons of scrapped Pokémon, including baby versions of existing monsters, unused evolutions, and all-new Pokémon.
Further digging has revealed that the overworld map was drastically changed in between the prototype and the final release. It appears that the Johto map (which is based of a small section of Japan) was once planned to include almost all of Japan. What's even more strange by far is the fact that Kanto was originally planned to be much, much smaller. You may remember hearing that Satoru Iwata's programming skills are what allowed Kanto to be included in its entirety in Gold and Silver. The original plan, it seems, was to condense all of Kanto down into one single city, located in the bottom right of the Gold and Silver map:
The game's original plot hasn't yet been fleshed out, but Resetera users have summarized the early parts of the game. It seems it was originally closer to Red and Blue in its introductory moments.
"The protagonist’s brother (named Ken) makes an appearance, ending the rumors that Gold’s brother was Red. You get an email from Professor Oak telling you to go to his lab, then your rival approaches you outside your house and asks you for what you call your mother (this was semi-unused in the final game, it’s used to prevent the Old Man glitch by storing names during Dude’s catching tutorial). You walk into the grass and Green stops you, bringing you to the lab like his grandfather did years prior. Green and Oak ask if you two want your starters, and then Oak takes you into the back room where the starters are located. You leave, talk to Green, and then Oak gives you and your rival Pokedexes. Your rival challenges you and unfortunately his team is missing so he challenges you with either garbage or things meant for demo trainers. If you somehow manage to beat him, you leave and your mother gives you 6 Pokeballs and the Pokegear. The script ends here and you’re left with the normal demo again." — The Shadow Knight
It's rare to get such a close look at a prototype like this, so enjoy! It's clear that Gold and Silver were majorly reworked in the two years after Space World. They ended up being extremely successful, and the Pokémon brand remains strong to this day.
Source: The Cutting Room Floor