Sneasel is a living fossil that tells us a lot about how Generation II itself was developed, and how the very concept of what being a Pokémon meant evolved over time.
You can’t make a game on your first try, and sometimes not even on your second. While developing Pokémon Gold and Silver, Game Freak went through several designs for Johto, which have now been leaked. Looking at how the world map was refined over the years tells us a lot about the evolution of the game as a whole.
An exploration of the development process for Pokémon Red and Green (or Blue), plus some info on beta Pokémon.