Ekans

The first-generation (Generation I) of the Pokémon franchise features the original 151 fictional creatures introduced in the 1996 Game Boy games Pokémon Red and Green.

The following list details the 151 Pokémon of Generation I in order of their National Pokédex—an in-game electronic encyclopedia that provides various information on Pokémon—number. The first Pokémon, Bulbasaur, is number 001 and the last, Mew, is number 151. Alternate forms that result in type changes and Mega Evolutions are included for convenience.

All 151 Pokémon from Generation I were designed by Ken Sugimori. The majority of Pokémon in this generation had relatively simple designs and were highly analogous to real-life creatures including but not limited to: Pidgey (a pigeon), Rattata (a rat), Ekans (a snake), and Seel (a seal). Many Pokémon in the original games served as the base for repeating concepts in later series.

Pokémon Sun and Moon introduced "Alola forms" of various generation I Pokémon. These versions are to represent "the different microclimates in the Alola region" where Sun and Moon take place. The Alola versions of Pokémon like Vulpix and Exeggutor have different appearances and types and were introduced alongside generation VI Pokémon. Alex Hern of The Guardian suggested that the developers likely decided to redesign various generation I Pokémon because, according to him, "fan connection with the original 150 Pokémon is as strong as it ever was, [while] the number of people who can tell a Pancham from a Swirlix is much smaller."

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Wikipedia