Sorry. With Darmanidude's question, my curiousity started running. Maybe a picture, but that would be hard.
Here is the actual sprite from Red/Blue. It is essentially just a bunch of random pixels.
An artist's rendition:
Um have you been to lavender town? He either looks like a ghost pokemon as far as i know.Also it's a hack so it doesn't have a definite form. i have seen a missing No look like a khangashkhan(hehehehhe bragging gengis khan) granted this was youtube but a surfing kangashkhan is pretty rare.
I know this bit. In every game his form has changed and like pokemaster said he is a glitch and having him causes more glitches. He has more forms than anything else and they all look completely different.