Hmm. I thought it meant Missing Number, as in that game Number is shortened by No. I've been recently messing around with Missingno, and have discovered a few interesting things. First, The actual picture that is Missingno changes when the surrondings and pokemon change. It's defense never rises from 5. If you confuse it succesfully, and it hurst itself, it will kill itself. Also, if caught it screws up the way you appear, as well it reverses the picture of your opponent.
There are two actual Missingno, The one called missingno, and the one with a block, M, and another block. These are different. You cannot put the second one in your box. If you do, you cannot withdrawn it again. The other Missingno can be. I have not looked into the first missingno, mainly because without a rare candy I cannot do anything with it. However, the second one, at level 0, can be leveled up. It will, at every level try to evolve into a Kangeskan. I'm not sure, but if you battle with it and do not get enough to level it up, it goes straight to 100. The easiest way to do that is to fight another one. Also, this is the only kind that doesn't kill itself through confusion. Another wierd glitch, is when I first got the second kind, I let it evovle to level 100 by accident and become a Kangashkan. I put it in the box to see whether I could take it out or not, and I could, except it leveled down to 70 when it did. However, unlike every other pokemon that I have ever seen lose levels, it kept it's health at where it was when it was at level 100. The bar actually went off the screen. I'm trying to level up the missingo naturally, while not letting it evolve. If I get anything else, I'll let you know...