From Bulbapedia, the community-driven Pokémon encyclopedia.

In each Pokémon game of Generation I, all Pokémon have an unseen index number used to identify them in the game's ROM. Unlike later generations, the Pokémon are far from being in order by their National Pokédex numbers, and most evolution families are extremely fragmented. In addition, 39 copies of the glitch Pokémon MissingNo. permeate the list, causing the last non-glitch Pokémon, Victreebel, to be located at index 190 instead of the expected 151. Index 0 and indices from 191 to 255 (hex 0xFF) contain all other glitch Pokémon in Generation I. Since Rhydon is located at index 1, many glitch Pokémon will become it upon capture and viewing of their glitched Pokédex entry, known as the Rhydon glitch.

When connected to a Generation II game, many of the glitch Pokémon, including each of the 39 copies of MissingNo., will appear to the other side to be a Generation II Pokémon and are occasionally tradeable. In the list below, the name of the Generation II Pokémon in parentheses after the glitch Pokémon's name is the equivalent Pokémon in the Generation II games. The conversion of Generation I glitch Pokémon index numbers to Generation II index numbers simply is Johto Pokémon in National Pokédex order, starting from 0xBF (dec 191)—the index after the last valid Pokémon in Generation I—and ranging to 0xFA (dec 250), then continuing by filling the indices that correspond to MissingNo.; only Ho-Oh is misplaced (Celebi is absent).

List of Pokémon by index number

Please note that the Ghost, Kabutops Fossil and Aerodactyl Fossil MissingNo. are not of type Bird/Normal and use the same type as the last Pokémon in memory.

Garbage data (glitch Pokémon)

Trivia

In an interview with Shigeki Morimoto, he confirmed that there were originally going to be 190 Pokémon in Generation I.[1] This matches exactly with the fact that the last non-glitch Pokémon occurs at position 190, and strongly implies that the 39 MissingNo. are formatted Pokémon data. Although each of the copies of MissingNo. can appear as or become a Generation II Pokémon when traded there, these Pokémon are simply based on the Pokédex order, so are not indications of the original Pokémon in these slots.

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