Glitch Pokémon

A glitch Pokémon is a collection of data that, due to programming oversights, a handles as if it were a  species.

Some glitch Pokémon have partially defined characteristics, such as "MissingNo." or "". Another example of this is the learn-set data for a Generation IV, where it learns as a starting move, likely to prevent the game from crashing due to an invalid move being used.

In Generation I, a few glitch Pokémon are hazardous to the game, some glitch Pokémon may freeze the game if sent out on the opponent's side, and a few may even corrupt save data, depending on their Pokédex number and the version used. Note that glitch Pokémon with index numbers greater than 199 will usually bring up glitch Trainers if encountered directly in the wild. These glitch Pokémon cannot be obtained via a wild battle, and must be obtained using a different glitch from the or old man glitch, such as evolution from another glitch Pokémon, trading an equivalent index number glitch Pokémon to another version, or by executing the Time Capsule exploit.

Conversely, in later generations, Glitch Pokémon are usually not hazardous, and generally only exist to fill empty space. The exception to this rule is the Bad Egg, which was added to the game to deter cheaters. Also, unlike the Generation I glitch Pokémon, which could be fought and captured using simple glitches, later Generation glitch Pokémon can generally only be seen by cheating.

Additionally, catching certain obtainable glitch Pokémon in Generation I may cause endless garbage Pokédex data to appear. To avoid this, the player must have activated a specific flag, if this is a hybrid glitch Pokémon, the flag is activated by either seeing a specific Pokémon or owning a specific Pokémon. Specific glitches (like the move or the ZZAZZ glitch) may cause the saved file to be corrupted or erased.

Trivia

 * There are 39 MissingNo. with identifiers in between real Pokémon, which would add up to 190 Pokémon.
 * Shigeki Morimoto confirmed that there were once 190 Pokémon planned for development in Pokémon Red and Green. MissingNo. is partially derived from blank entries in the various Pokémon data arrays. Most of its characteristics are derived from zeroed values in these arrays, including the pointer to its glitched base stats.
 * The reason for the large number of glitch Pokémon in Generation I is because only 151 values correspond to valid Pokémon, while Pokémon species index numbers are stored in 8 bits allowing for up to 256 values. This leaves 105 values not corresponding to a valid Pokémon; which, excluding the 39 MissingNo., leaves 66 different values for other glitch Pokémon.
 * In Generation I, a large number (43) of glitch Pokémon have the small character ゥ u in their names. This is because it corresponds to a common codepoint in Game Boy assembly code.
 * The only other Japanese character that appears somewhat frequently in glitch Pokémon's names is small ァ a. The hiragana ぉ o and ぇ e also appear, but less often.
 * The reason for any Japanese text in an English game can be seen here.
 * 23' 0"/880.6 lbs. are the most common height and weight values for glitch Pokémon, and  23' 2"/1031.6 lbs. for . It is unknown as to why any glitch Pokémon have the same values, however.
 * Some glitch Pokémon have four or more evolutionary stages, more than any valid Pokémon.
 * Encountering a MissingNo., or a few other glitch Pokémon, will cause the screen to go blank for a small amount of time before the battle begins.
 * The same also occurs when they are sent out in battle, but the game will buffer on the battle screen rather than a black screen.
 * Glitch screens also sometimes appear while the game is waiting to send the Pokémon out.