Actually no. Arceus existed before Mew. It was the first pokemon and created the universe.
Then it created other legendaries.
Mew was the first pokemon to naturally evolve into existence, from single-celled organisms, and every non-legendary afterwards evolved from Mew.
Bulbasaur is the first in the dex, like you said.
Rhydon's dex entry does NOT say it's the first. But it was the first pokemon to ever be designed (in real life, outside of the games)