I found the prototype, or concept if you will, of the first Pokémon game a while ago in Bulbapedia.
Instead of being a leech I’ll link you to the page:
Capsule Monsters – by Bulbapedia
My favourite concept drawing of the game is this one:
It depicts Red (the player character), with an early version of Rhydon with him, exploring a cave which contains a hatching unnamed prototype Pokémon that has never been seen anywhere else but in this picture.
In my opinion this picture catches the exploring and adventure feel of the first six games the most.