From Bulbapedia, the community-driven Pokémon encyclopedia.
The Rhydon glitch affects glitch Pokémon with an invalid Pokédex number in Generation I when the Pokédex entry is shown upon catching it. All Pokédex numbers from 001 to 151 contain official Pokémon, so are valid.
After the Pokédex entry is displayed upon catching a Pokémon with an invalid Pokédex number, an affected Pokémon will become Rhydon, as a subroutine used when giving the player a Pokémon converts all invalid Pokémon to Rhydon. The Pokémon's moves will be replaced by Rhydon's moves at the level it was caught at. If this Pokémon is sent to Bill's PC, it will become an unstable hybrid Pokémon, with the original glitch Pokémon as the recipient and Rhydon as the donor. Due to all hybrid glitch Pokémon having Pokédex numbers equivalent to official Pokémon, they are not affected by this glitch.
Returning the glitch Pokémon to its original self
To return the unstable Rhydon-glitch Pokémon hybrid back to just the original glitch Pokémon, it must be stabilized. To stabilize an unstable hybrid Pokémon, a Pokémon can be evolved (in the way the glitch Pokémon would), traded to Pokémon Stadium 2, or deposited and withdrawn from Pokémon Day Care. However, the glitch Pokémon will still have the nickname "RHYDON" after being stabilized. This allows these glitch Pokémon to learn every move Rhydon can. Once stabilized, the Pokémon can be traded to the Generation II games.
Methods of avoiding the Rhydon glitch
- Obtaining a glitch Pokémon with a Pokédex number less than 152 which is not 000.
- Obtaining a glitch Pokémon which doesn't show its Pokédex entry upon capture.
- Activating the flag which is the same as the obtained flag for the glitch Pokémon as a method of avoiding the Pokédex entry. For example, Pokédex entries belonging to glitch Pokémon in the No. 000 family (such as Missingno.) will only appear upon capture if the player has not encountered Cubone.
- Obtaining the glitch Pokémon by evolution.
- Trading the glitch Pokémon from another version to obtain the equivalent glitch Pokémon.