Bulbasaur is the first Pokémon in the original Pokédex listing.
Its National Pokédex number is 001, making it the first entry in the 151-species roster introduced by Pokémon Red and Green. Bulbasaur is a dual Grass/Poison-type Pokémon and is described as carrying a seed planted on its back at birth.
Bulbasaur is also one of the three selectable starter Pokémon in the original games, alongside Charmander and Squirtle. Its position in the Pokédex is not based on its popularity or on its role in the anime; it is the first numbered species in the official sequence.
Pikachu is often mistaken for the first Pokémon because it became the franchise mascot and stars prominently in the anime. Mew, meanwhile, is number 151 and was treated as a secret or special Pokémon in the original generation. The numbering distinction helps separate franchise prominence from Pokédex order.