Only Ken Sugimori knows that, but we can guess.
Since Bulbasaur is the first Pokemon in the National Pokedex, it's most likely him. In fact, Pikachu wasn't originally intended to be Pokemon's mascot at all, it just came to be through the media.
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Arceus was the Pokemon who created the world and the first Pokemon in it Mew therefore Arceus came first but still leaving Mew the first Pokemon.
Satoshi Tajiri
It came out in Japan in 1996. It came out in America in 1998.
Definately the original Mario. Then Zelda, then Pokemon.
From what I've read, The Pokemon Games came out first.