Missingno. is a pokemon eldritch abomination, plain and simple.

How?

It comes from seemingly nowhere out of the ocean to inflict it's unintelligible mass of garbled horror upon your character's mind, breaking it in the process of trying to understand what he's seeing. This is shown, as the glitch has side effects after seeing it, like the mangled up hall of fame and somehow believing he has infinite copies of some item in his bag. Furthermore, when you meet him in the later versions, Gold and Silver, the poor guy's been rendered so insane that he can no longer speak, and has become a recluse in a cave.

...Well that's my theory anyway. XD ;