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Venusaur is listed in the pokedex as the Seed Pokemon. It is so because it is the first fully-evolved pokemon in the pokedex, and the strongest evolution line in the early-game. It is the seed that allowed the franchise - and the player's adventure - to grow.
Charizard is listed as the Flame Pokemon. It is so because from the very beginning, it was always one of the most popular pokemon available. Kids all over the schoolyard would kill to get a holo-foil Charizard card, and if you didn't own at least one Charizard in-game, you were guaranteed to be bullied. It is the flame, the passion, that kept the franchise alive and blazing.
Blastoise is listed as the Shellfish pokemon.