Damn, there are some layers here I had to check the comments to get:
These are the four "first" pokemon. Mew's pokedex entry used to call it the ancestor of all Pokemon. Rhydon was the first Pokemon designed for the games. Bulbasaur is #1 in the Pokedex. Arceus is like God in the Pokemon universe, I'm told, and in Pokemon lore created everything in that universe. There's a meme where a very distressed Ash is asked "What is the first Pokemon?" and given these four as the answers.