Missingno.

Description
Representative of a side of the Pokémon world few dare to explore, Missingno. is an enigma. Exceedingly rare, it has eluded even the most prestigious Pokémon researchers; as such, very little in the way of official information on it is available. Missingno. possesses immense strength which it can focus into its physical attacks. Despite lacking any semblance of limbs, Missingno. is capable of punching. It also possesses psychic powers.

Volcanic islands are one's best bet for locating Missingno. They are commonly seen flying above the water's edge, though never actually in the water.

Location

 * Mystery Gift v6.2.1: s40rf534
 * Mystery Gift v0.8: sndgames

Shiny

 * Mystery Gift v6.3.1: 46233nc4

Shadow

 * Game Corner (Random Non-evolved Shadow Pokemon): 300,000 coins
 * Mystery Gift v6.5: s20r25o4

Trivia

 * Missingno. is one of the few Pokémon not listed on the Pokedex. The others are Entei, Raikou, Suicune, Kyogre, Groudon, and Victini.
 * Missingno. is the only Pokémon that had all 3 varieties (Regular, Shiny, and Shadow) available via Mystery Gift.
 * The normal Missingno. is the only pokemon that appears twice in the mystery gift codes
 * Missingno. is the final Mystery gift available in the game.