It can only be obtained by the Time Capsule exploit 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 back sprite has a glitch screen, particularly the blackout variant.
This glitch Pokémon article is part of Project GlitchDex, a Bulbapedia project that aims to write comprehensive articles on glitches in the Pokémon games.