|Bulbapedia also has an article about Glitch Pokémon.|
|This article is a summary page for different variations of a glitches, etc. when talked about as a whole.|
Glitch Pokémon (Japanese: バグポケモン, Bug Pokémon) can refer to any type of Pokémon with glitched traits or traits unintended by the game developers.
It can also refer to a Pokémon which is the result of force-loaded data as Pokémon from locations in the game's data structure (such as non-Pokémon related structures in the ROM or RAM, it could also include a glitch Pokémon with an intentionally designed sprite but which is invalid in other forms) not intended to be loaded as trainable Pokémon.
Glitch Pokémon species
Commonly the phrase glitch Pokémon refers to force-loaded Pokémon data for a Pokémon index number which isn't used by any valid Pokémon. These invalid Pokémon can be accessed by a glitch or by using a cheating device.
There can be a number of reasons why glitch Pokémon behave the way they do.
Some of the data for MissingNo. (specifically its Pokédex number 000, name, three of its sprites (Kabutops Fossil, Aerodactyl Fossil, Lavender Town Ghost), Super Game Boy palette, Pokédex entry in Japanese Blue and nine of its cries) are believed to have been intentionally programmed at one point but left in the game by the developers, becoming unused in normal gameplay.
However, most of MissingNo.'s data (including regular sprites, types, its starting moves, base stats, menu sprite, catch rate constant, experience group and TM/HM learnpool) are unintended (referred to in programming circles as "garbage" data), and are the result of extrapolated data. The aforementioned data being unintended also applies to the remaining 66 Pokémon (Pokémon IDs 0x00, and 0xBF through to 0xFF), which also have unintended names, level-up moves, Super Game Boy palettes, Pokédex entries.
Specifically this means the game engine looks up the Pokémon's index number and Pokédex number (see glitch Pokémon family) and uses it to calculate where the Pokémon's data is located, which for glitch Pokémon can be beyond the end of the valid Pokémon data structures. This then results in data encoded as the Pokémon data (e.g. as a sprite, a list of moves it can learn, base stats) when it was meant to be used and read in another form (consider the analogy of opening an MP3 file in Notepad; the result is unintended or garbage text).
Most glitch Pokémon in the first two generations have indecipherable front or back sprites, with the exceptions of some hybrids and the fossil and ghost Missingno. (which have valid front sprites but invalid back sprites). Every glitch Pokémon except for Missingno. has an invalid name, that may include glitch characters.
In the third generation, Decamarks all have the "unknown Pokémon" sprite seen in the Pokédex (although it may be corrupted, depending on the index number) and are named ?????????? on the opponents side. Not all glitch Pokémon in Generation III are named "??????????" on the nickname or summary screen. Here, they can have names like shortened versions of move names such as "RBOLT" (instead of "THUNDERBOLT").