Missingno, as well as the DPbox. What's the DPbox?
This.
Now, you may say, "But Will, it's just a bunch of freaked out pixels! What's so scary about that?"
Think about it, seriously think about it, from the trainer's view. You're content roaming around and catching pidgeys when suddenly you see this... What is it? It seems to exist both in and out of reality at the same time. Its blocky form seems to be comprised of a twisted mockery of things you can just barely recognize, but at the same time its static nonsense is completely alien to you. You've discovered a sentient hole in the fabric of your universe.
And as you watch, it spreads like a virus. Other pokemon it contacts are similarly scrambled to horrible jumbles of body parts and chaos. And somewhere deep in the ocean, there's an entire island where nothing but this mind-shattering chaos reigns for miles. In realistic terms, Missingno is an incurable extradimensional parasite that will not stop until all the world is consumed. This is death and madness incarnate. No fire, no brimstone. Just this wall of simultaneous nothing and everything. And you're face to face with it, pitting your poor, doomed pets and friends against it.
And then your world ends and your mildly inconvenienced god has to go and buy a new one.