MissingNo., 'M, and the other "glitch" Pokemon are actually Lovecraftian Cosmic Horrors.

Consider the following:

• The "glitch" Pokemon generally appear as disturbing forms that cannot be properly perceived by humans (except for MissingNo., which occasionally appears in the equally-disturbing forms of a ghost or an extinct Pokemon's skeleton). [*]Reality behaves strangely around them. The most benign form of this is the Rare Candy Cheat, which duplicates one of the items held by the player when he or she encounters MissingNo. The more malevolent Charizard 'M transforms other Pokemon into copies of itself. Finally, encountering some "glitch" Pokemon will immediately warp the trainer to "Glitch City," a twisted and labyrinthine parody of a real city from which it is impossible to escape unless you can fly. [*]Trainers who encounter them tend to either go insane or die horribly. This one requires some explaining. Some "glitch" Pokemon cause the music or graphics of the game to distort, either in the overworld or in specific places like the Hall of Fame. Others corrupt the game so badly that they can crash the game or render the save file unplayable. Now, consider that everything in the game takes place from the POV of your character, the trainer. What does seeing and hearing things differently from everyone else sound like, if not insanity? And the game crashing, forcing you to restart from an earlier point in time, is similar to the way most other video games treat death (especially notable because it's impossible to die any other way in a Pokemon game). Finally, the corruption of a saved game file, which forever ends that character's journey by leaving him or her unable to interact with the world, is likewise the in-game equivalent of death or incurable, crippling insanity. [/list:u:727fad6d24]

  Thus, the various "glitch" Pokemon are reality-warping horrors whose very presence twists the world around them. In fact, given MissingNo.'s multiple appearances, it could actually be a form of Nyarlathotep; after all, those who encounter MissingNo. and receive its item duplication boon are likely to chase after other "glitch" Pokemon as well, and even those who don't risk seriously destabilizing the Pokemon world by abusing their nigh-endless supply of Rare Candies and Master Balls. Both would fit perfectly with Nyarlathotep's goals of spreading chaos and insanity.