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When asked "which Pokémon is the first one?" without any further context, five answers are valid:

Bulbasaur :bulbasaur: is number #001 in the National Pokédex
Arceus :arceus: is a deity in the Pokémon mythology it hatched from an egg then cerated the universe
Mew :mew: is the common ancestor of all non-Mythical Pokémon, and probably most mythical Pokémon as well
Poliwag :poliwag: was the first Pokémon created by Satoshi Tajiri that ended up in the games. It also has #001 in the Pokédex assigned in early concepts
Rhydon :rhydon: has the internal ID 0x01 in Generation 1 (0x00 is the glitch Pokémon 'M, often mistaken for MissingNO. because it uses the same sprite, but it's not an official Pokémon)