It's creepy and if it were a pokemon it should be a man made experiment like mewtwo or porygon that went horribly wrong. missingno is ugly, no pokemon should look like a deformed block of pixles.
Hey, another good idea for the origins of Missingno. It isn't even natural! It's a man-made abomination, an experimental freak gone horribly awry! Probably made on Cinnabar Island; it was a byproduct made when the computer programs used on the Mewtwo project accidentally fused with the computer-recorded DNA sequences extracted from Kabutops and Aerodactyl fossils, and the programs, designed to bring actual life to actual DNA cells, brought digital life to the digital DNA sequence.
A single Missingno escaped from the computers at the same time Mewtwo escaped from the now-destroyed mansion, and while Mewtwo fled north, Missingno fled east, and made its home along the Seafoam Islands coast. Ever since, it's been multiplying like a virus, and occasionally mutating into new forms. Its digital history also explains why it interacts badly with some computer programs found in average PCs, like the Wall of Fame one. ("MISSINGNO used GLITCH POWERS! WALL OF FAME became confused!")
Perhaps a flock of Missingno once devoured a Rhydon, returned to the main pack, and dispersed the "data" they ate to the rest, which is why sometimes Missingno evolves into Rhydon: fragments of Rhydon DNA is in its programs now.
Later, Silph researchers were able to discover the coding processes in the Cinnabar computers that Missingo "evolved" from; Silph Co. modified them and used them to create Porygon.
(Can you tell I like coming up with these ridiculous stories? I think I like the first one better, but this one is fun in its own way. Call the first version "Missingno, according to creationism," and this second version "Missingno, according to evolutionism.")