Missingno. the glitch pokemon found in Red or Blue version, was a dropped out pokemon after Nintendo (Or another company involving Pokemon) saw it.
But people never knew what it was.
Most people, like me, say that it was a Grass-Flying type Legendary bird, which makes total sense, because they have fire, water, and lightning, but no grass, one of the main types. The company must have thought it was badly drawn, or just a crazy idea. But, a grass type Legendary Bird does make sense.
Some people know about Ghost, another pokemon related to Missingno, that knew only one move that one-shot every pokemon. But, after you beat the Elite Four, your game crashes, and you have to face him without any pokemon when you start, making you lose.
So, what is missingno?