The game was originally going to be called Capsule Monsters or Capsumon, but the name was unable to be trademarked which led to it being revised to "Pocket Monsters" which was shortened to Pokemon.
As we can see, this early Lapras design heavily represents the one we ended up with. The final result just looks a little less bland and dinosaur like.
Some early battle concepts showing Slowbro and Gastly as well as Blastoise and Staryu.
Here's that same scene. Note the whips the trainers carry, which were later omitted from the final battling concept but still included in certain sprites like Sabrina and the Rocket Grunts.
Early concept of a Pokemon town. Note what appears to be a Primeape in a cage on the left page.
A trainer exploring a cave with his Rhydon (a slightly earlier design). Notice the Pokemon hatching from an egg down the bottom of the picture. This shows that the concept of breeding/hatching Pokemon eggs existed long before Gold and Silver.
Too bad the fantastic 'monkey in a hat' didn't see a final release. We can only imagine that it went on to inspire Primeape/Mankey. The left looks a little like Lickitung or Kangaskhan.
Is that Charizard?
Early map of the Kanto reigon. The numbers likely represent where the gyms were originally going to be located, or just the order in which the player would visit the areas.