is a Glitch-type Glitch Pokémon found in Pokémon Red and Blue Versions.
It can only be obtained by the Johto guard glitch with Yanma. When it is sent out by an opponent the game will crash, however the game will not crash if it is sent out by the player.
The :L in its name is actually a changing tile, that references tile 238 at the character base 0x8800, and therefore is defined as a glitch block.
's sprite has a glitch screen, particularly the blackout variant.