From Bulbapedia, the community-driven Pokémon encyclopedia.
A Glitch Pokémon is a Pokémon that exists in a game due to a programming or level design oversight or for beta testing, and as it was not intentionally placed inside the game by Nintendo or Game Freak, it might be hazardous to saved data. They were originally supposed to be place-holders for unused Hex-adresses in the game. Glitch Pokémon are supposedly to be avoided due to the risks they may pose to a player's save file (in reality, many of them (though NOT all of them) are absolutely harmless).
This is a chart of all 24 known glitch Pokémon that can be found in the wild in the Red, Blue and Yellow versions without using a cheating device:
The following glitched Pokémon can be found in the Gold/Silver versions, but require Gameshark to complete them. (Note: after capturing these Pokémon, the game's graphics have been known to screw up, and the player will find themselves in a Glitch City-like area.)
- The Yellow version Missingno. is different from the Red/Blue Missingno., but share the same name. For the Yellow version Missingno., using a Pokémon in Method #3 of the Mew glitch with a special stat of 31, 32, 61, 62 , 63, 67, 68, or 69 would make it appear(). Missingno will revert to level 100 if leveled up to over level 255 with Rare Candies or if it fights in battle and wins.
- The Pokémon 3TrainerPoké, Charizard 'M and Q are the only known catchable glitch Pokémon that cannot be directly caught using the Mew glitch.
- The glitch Pokémon ".4" can corrupt saved data if it uses a certain glitch move, making it impossible to continue and forcing the player to start a new game.