>>38249834It's not totally set in stone that Rhydon is the first-designed Pokémon, but a lot of things point to it.
The Pokémon were all designed as pixel art before being illustrated by hand later on, and were inserted into the game in the order they were made. They were probably just the product of Tajiri and Sugimori spitballing as Tajiri laid out his idea for a new game about monsters.
The original Capsule Monsters document was made once these sprites were created.
The sprites listed here show Rhydon as 0, although it's a bit out of order from the index numbers in the games. The index numbers got rearranged a lot during development so they're not indicative of everything, but Rhydon probably stayed as #1 the whole time.
Ken Sugimori also said that the first Pokémon were "Rhydon, Clefairy, and Lapras." He was oversimplifying things in that statement (there were more than just those 3) but there's also the fact that the statues in the gyms and the Substitute Doll used to look like Rhydon as well.
Even if there were rough ideas before Rhydon, it's safe to say that it was the first "real" Pokémon as far as the Game Freak OGs are concerned.
