Think of a white circle. Now put a black outline around it. Now put between 1 and 3 question marks inside this circle. That's Missingno. 1 and 3 question marks are bad glitches that apparently cause bad eggs, but 2 question marks aren't. You can catch the 2 question mark one, but it only knows Tackle, and if you try to teach it any moves, the game glitches up.