ok, i dont know for sure, but i think Missingno is just a test Pokémon that the programmers fergot to take out. it can also be that the programmer's messed up Missingno's stats (like bird type when its rilli supposed to be flying type) and redid the stats but fergot to take the messed up part off.
Missingno stands for missing number, at least i think. i just *think*, but dont know. i also think Missingno might originally be one of the legendary birds but got messed up (he's at level 80 and so r the other legendary birds[or r they at level 70???]) anywayz, Missingno is just missing data that was never meant to be accessed, but by doing the Missingno cheat, it can be.
i caught Missingno once and it messed up my battle images and Hall of Fame. when its in ur party, it will have the icon of a character turning into an octagon (8 sided shape). it will have 2 water gun attacks and a sky attack. i dont know anything about the bird type, tho. anywayz, Missingno is perfectly harmless (unless u count the messed up battle images and missing Hall of Fame) to my knowledge.
i've used the cheat many times and it never did anything. one time i caught a 'M (different kind of Missingno). i deposited it in my Pokémon box and it froze my game, but that was the only time there was anything wrong. if u dont believe me, try it urself. dont blame me if ur game reacts differently, tho. happy hacking!!! :)