A demo of Pokemon Gold And Silver contains a large number of unused Pokémon designs. The demo file surfaced over 20 years after it was originally playable at the Nintendo Space World trade show in 1997, according to The Cutting Room Floor.
Fans at The Cutting Room Floor analyzed the demo's files, revealing the unused designs. These include old designs for existing Pokémon, such as the legendary beasts, Raikou, Entei, and Suicune.
There are also completely original Pokémon, like a vine monster called "Gelanla," a spooky hamster-cat called "Electiger," and several unused "baby" evolutions, like a pre-Meowth Pokémon called "Meowsy."
ResetEra user The Shadow Knight compiled an image containing every sprite in the demo, according to IGN, including the original 151 Pokémon and the unused second generation designs. Some highlights include an unused evolution for Farfetch'd, an awesome unused Hoothoot evolution (replaced by Noctowl), and two unused starter Pokémon.
A full list of the Pokémon's names and their accompanying images can be found at The Cutting Room Floor. More details about the demo's files can be found in a spreadsheet assembled by the team who sorted through the files.
I'm very glad some some of these designs didn't make it to the final game (Raikou's in particular looks like a bad Neopet), but some of them are pretty cool. I wish that the unused Politoed design (the frog-thing on the far right of the second-to-last image) hadn't been replaced with the current Politoad. It's cute and all, but there's something really awesome about that big goofy frog design. I hope Game Freak brings some of these to the games eventually, because a lot of them are way cooler than an ice cream Pokémon.
Source: Game Informer Pokemon Gold And Silver Demo Reveals Unreleased Pokémon