There were originally going to be 190 Pokemon instead of 151; 39 of them were later moved to Pokemon Gold and Silver. By checking the game's code, you can see the orders in which the Pokemon were entered into the game, Rhydon being first and ending with Victreebel. Since Ken Sugimori, the designer of the Pokemon, confirmed that Rhydon was the first Pokemon ever designed, it can also be inferred that the Pokemon were put into the game in the order in which they were originally intended to appear in the Pokedex. The ordering also proves that all 190 Pokemon were conceived before the original games' release.
Contributed by game4brains