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.