I think the Wobbuffet theory that Zayphora mentioned is more likely. I mean, taking into account that Wobbuffet actually is mostly used as a decoy, or at least for protection purposes with Counter and all, it'd make sense that the main part of it was just a kind of shield to defend itself with, as its actual body is weak and defenseless.
That's not to say that the MissingNo theory isn't possibly, it just wouldn't be as likely to be the truth. There's just one really important thing you have to remember, and that is, that MissingNo isn't anything more than a glitch. Of course the fanbase has built on to the origin of MissingNo, but they're all fanmade theories or stories, and MissingNo doesn't really play as anything more in the Pokemon world than an, at the first time around, a widely spread rumour, which was later proven to be true, however just a glitch. The second generation came relatively close to the first generation, and there had probably been enough time to give MissingNo a lot of fame, however, the first thing the developers think about is probably the Pokemon. Backstories and such may not be thought out before late into the development, however the designs probably come early on, which would probably mean that Wobbuffet was designed before MissingNo got so famous, kinda overruling the theory.
But alas, it's just a theory. It could very well be true, but these are just my thoughts on it.