Glitch Pokémon

Glitch Pokémon are Pokémon in the video game series that are the result of scrambled or leftover data that can be found by use of cheat devices, glitches, and other ways. Glitch Pokémon are not intentionally placed inside the game; they consist of data that the game reads incorrectly under certain circumstances. Most of them cause no harmful or severe damage to a save file, but some are known to trigger negative effects that can be potentially irreversible. These effects include save-data corruption and/or deletion.

Explanation
In the Pokémon games, a variety of variables are used to identify the species of a Pokémon. During a battle, for example, variables in the game's memory would store the species of your Pokémon and your foe's Pokémon. Glitch Pokémon exist because these variables are (by necessity) too large.

In Generation I and II games, the variables used to refer to a specific Pokémon are large enough to hold 256 possible values (including 0); this is because variable sizes must be powers of two, and the next smallest size is 128 values, which would not have been sufficient. A similar situation exists in Generation III and IV games, with the variables each being able to hold 512 values (again, including 0).

In all cases, those variables are capable of referring to more Pokémon than there actually are in the game. When such a variable is set to refer to a non-existent Pokémon, the game ends up treating non-Pokemon data (such as graphics, music, and program code) as Pokémon data.

The effect is that in Generation I games, there are 105 glitch Pokémon, including duplicates and not including "hybrids". In Generation II games, there are exactly 5 glitch Pokémon (including EGGs). In Generation III games, there are 101 glitch Pokémon (because Unown's twenty-six forms count as twenty-six separate species, forming a total of 411 non-glitch Pokémon).

Pokémon Red and Blue

 * MISSINGNO.
 * "MISSINGNO." actually refers to several glitch Pokémon who share a name.
 * 'M
 * "'M" actually refers to a handful of glitch Pokémon who share a name.
 * LM4
 * A
 * a
 * ゥ or ゥ|Chiisai-u
 * .4
 * Charizard 'M
 * h POKé

Pokémon Yellow

 * MISSINGNO.
 * Note that Yellow 's MISSINGNO. varies substantially from the one(s) found in the Red and Blue versions.
 * 3TrainerPoké
 * ♀
 * X - x
 * Q
 * 7g
 * 4..
 * P pkmn
 * Z4
 * 4CH4hi (encountered by glitch Misty only)
 * 44Hy

Pokémon Gold, Silver, and Pokémon Crystal

 * ?????
 * "?????" actually refers to four glitch Pokémon who share a name.
 * EGG

Pokémon Ruby, Sapphire, and Pokémon Emerald

 * ?????????? (colloquially known as "Decamark")
 * ?
 * "?" refers to several glitch Pokémon who share a name.
 * Bad egg
 * B óË ÁN

Pokémon Diamond, Pearl and Platinum

 * Bad egg
 * DPBox
 * Invisible Shiny Bulbasaur
 * Missingno (possibly in older releases)

Gallery
The following images show some glitch Pokémon and some of their effects.