People may wonder "Why does MissingNo. and other glitch Pokémon have glitched sprites?" and i found out something useful. I have this program called YY-CHR which is able to open up most ROMs and view/edit their graphics. I opened up Pokémon Red and scrolled though the tiles to find random pixels in most of the tiles, I did find the tiles that are in the game viewable but hidden throughout the glitchy tiles. I got interested and reversed all of the tiles only to find that Visual Boy Advance Didn't like it (or the ROM hated it's self) and the emulator crashed. So those glitchy tile might be some sort of code that the game runs on. I found Ash's sprites and tried to change them to MissingNo. even though they hardly looked like MissingNo. the game ran fine (except that Ash looked like The Blob)
So that shows that MissingNo.'s sprite was a probably a part of the program.