explaining the joke
in lore, arceus is called the first pokemon that created all the rest
also in lore, mew is called the first pokemon as in the common ancestor of all pokemon
bulbasaur is pokedex number 1
and rhydon (not rhyhorn, oops) has index number 1 in gen 1, and is likely the first pokemon created during development
re: explaining the joke
@Felthry the first pokemon is pikachu because it's the first pokemon I had