In Game Maestro #4 he said the first batch of Pokemon were based on kaiju, and singled out Rhydon and Nidoking as two early designs that survived into the final game. Rhydon was the most likely to have been the very first, since it appears the most in pitch documents, and also had the lowest index number in development (0), as well as the games’ final build (1). But Sugimori likely made these sketches in quick succession, and maybe can’t remember the exact order of which Pokemon he created before others. In other words — Rhydon, Clefairy, and Lapras were among the first Pokemon, not necessarily the first three ever created.
The exact quote from Ken Sugimori was:
“Who was the first Pokemon ever created?”
Sugimori: “Rhydon, Clefairy, Lapras, etc. Our original idea was for Pokemon to live alongside humans and assist them in useful ways. So the earliest Pokemon had well-defined functions — like ferrying people across the sea or carrying things. I also created Pokemon like Clefairy who were basically cute pets.”
All evidence (including Sugimori’s statements) point to Rhydon being the first. This honorary place was continued with Rhydon being the substitute doll, appearing as statues in gyms etc.
https://lavacutcontent.com/sugimori-masuda-developer-interview/