List of Pokémon by index number (Generation I)
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 Dex numbers, and most evolution families are extremely fragmented. Just as well, glitch Pokémon, like Missingno., permeate the list, instead of being located in one solid block as they are in later games. A reason why many of the glitch Pokémon can be seen evolving into normal Pokémon is the fact that, unlike normal Pokémon, who have index pointers for their evolutionary lines, there is often no data, and the glitch Pokémon simply evolve into the first data they encounter. Since Rhydon is index number 001, many glitch Pokémon will become it upon capture.
