Missingno or Missingno. (けつばん ketsuban) is a Glitch Pokemon encountered in Pokemon Red/Blue. The name has been shortened from missing number, due to the Glitch Pokemon not having a Pokédex number.
It is of the Bird/Normal type, and uses 2 different cries: a short Nidoran♂'s cry in battle, and Rhydon's cry on the stat screen. The Bird type is thought to be a beta version of the Flying type.
Missingno's common form
The most common form of Missingno appears as 3/4 of a rectangle, and consists of various parts of the game's tile set. However, it can also appear as a Ghost (similar to fighting a Gastly without a Silph Scope), or as any other Pokémon. Missingno's level also varies greatly, however it commonly sports a level between 80 and 130.
Effects
Catching a Missingno can severely damage the game cartridge, or your saved data. It also affects the Hall of Fame entries. Missingno's cry and appearance are determined by the player's name. It cannot be traded any further than Pokemon Yellow, and even then the trade can encounter difficulties.
Reason
Missingno is the Glitch Pokémon that appears after viewing the Old Man's tutorial in Viridian City on how to catch Pokémon, then immediately Flying to Cinnabar Island and Surfing along the right hand side of the island until one is encountered, without visiting any other areas.
Whenever the game "sets up" the planned battle against the Old Man's Weedle, it needs to change the player's character name to "OLD MAN" so that it will display "OLD MAN" instead of the name the player has entered.
The programmers decided to use the area of data where wild Pokémon information is as a temporary storage area for the player's name, due to the lack of memory on a Game Boy. Normally this wouldn't cause any abnormal activity, as the correct data for the wild Pokémon available is written to this area in memory whenever the player travels to an area where it is possible to catch wild Pokémon.
Along Cinnabar Island's coast, however, there is no data indicating which wild Pokémon are catchable, and the same is true for both Viridian City and Cinnabar Island itself, at least by movement in long grass. The game uses whatever data was already in the corresponding area of data when determining which wild Pokémon encountered and their levels-- now the player's name. Normally this space in memory would hold the data of the last area visited where wild Pokémon were catchable in grass and their level data (this same glitch allows players to exit the Safari Zone and Fly immediately to Cinnabar Island to be able to catch and fight Safari Zone Pokémon in the same way as they would in other areas), however, since in both Viridian City and Cinnabar Island the data is empty for Pokémon obtainable in the grass, the active data is never overwritten, leading to Missingno's. availability, among other things.