If you trade Missingno to Gold, Silver, or Crystal (the new games that you can even trade from Red/Blue/Yellow with Missingno) it will usually become one of the new species (although with Missingno attacks), but there still is a chance it could remain just plain wack.
And if you trade Missingno to Ruby or Sapphire, it proceeds to rape May, apparently.
it COULD also be mew from pokemon red or blue. There was a bug that I used to get mew back then, and he looked like this. Forget if I could do shit with him though, I think it froze the game after you got him.