Wow, it's been what, almost 15 years since MissingNo first appeared, and there's still speculation about him?
Here is Nintendo's official statement:
MissingNo is not Togepi, or Lugia, or any real pokemon for that matter. MissingNo is a placeholder for when the hexadecimal data for wild pokemon is incorrect, thus generating an invalid pokemon. No secret Nintendo conspiracy here, just buggy programming.