(Made in August 29, 2010)

‘M or Missingno in it’s Decoy/Substitute form.
(I directly took the colours of ’M or Missingno (Since they had the same sprite in Generation I))

More infos from Bulbapedia;

‘M and Missingno. can be uploaded to Pokémon Stadium, and (if the game doesn’t freeze) will look like Substitute dolls [Pokémon Decoy] (small Rhydon[/Tyranitar] or Kangaskhan-like figures that appear when a Pokémon uses Substitute). ‘M’s graphic is blue and Missingno.’s is purple. When viewed in the Pokémon Center, all of ‘M’s stats will be question marks (including the Pokédex species ID number, even though the Game Boy versions identify ’M and Missingno. as #000). However, ’M will not be allowed to battle, nor will it function correctly in the Gallery (in which players can take pictures of Pokémon). Photos of it will not develop if taken.

If ’M or any other glitch Pokémon is selected in the “see list”, the game will freeze, playing the music without the bass track in the background. If a player tries to view ‘M’s stats, the game will also freeze on occasion.

Similar problems [During a link trade in Generation II “Your friend’s (‘M or Missingno’s name) appears to be abnormal”] present themselves when attempting to transfer ’M or Missingno. to Pokémon Stadium 2. If successfully transferred, ’M and Missingno. will become a Ditto.