I did the Missingno cheat. When I fed my Missingno a Rare Candy, it went..

Well, first flying onix is completely fake and you can't get one even with Gameshark. The reason you're Missingno doesn't evovle into Kangaskan is because you need 2 catch a lv.0 "m" and then use a rare candy. And for when it battled and went down to lv.100 is just the way it is.

Missingno is supposed to be a test module implanted in the game for research and development, and I'm guessing that because they evidently failed to remove it properly it resulted in a dangerous, potential glitch that can have a variety of effects. On the Blue cartridge I am convinced, after reading several testimonials, that Misingno practically exists to debilitate your game in one way or other, and what effects these may be tend to vary depending on the condition of and the events passed in your game, so nobody can actually confirm that. In that case the event that was supposed to have Missingno turn into Kangaskhan was instead an abnormal experience gain. What I can tell you that your Missingno's devolution is a reaction of the game to the application of experience, and this affects not just Missingno, but all Pokémon that occur at levels above 100; it's some sort of rectification mechanism. I'll tell you more once I sort this out.

Perhaps it was a glitch.

Missingno, being a test subject and a debugging mechanism in the game, can cause unpredictable results, as you may know, and because of this, whatever the results are in response to what you do with Missingno are totally dependent on your current condition and position in the game. Thus, nobody can confirm what may happen in one situation or another. The conditions I have experienced so far have done nothing to debilitate my game in any way, but everyone's game is different, and depending on your position, and the data layout as a result of your choices in the game, will impress upon a certain result, in one form or another on your game. Thus, there is no precise answer to solve your problem.