Well... every Pokemon really, provided we're on about Green Version's sprites.
It really depends how they're drawn on whether they're even scary at all; I've got a nice image set with all Pokemon up to gen 2 drawn in traditional Japanese painting style and some are proper freakish, and burly ones like Ursaring would be mighty intimidating to encounter.
I'm sure if Pokemon were real, we wouldn't have children combating them, since even ignoring their appearances, many of the older ones' original dex entry descriptions are quite sinister.
Kinda amusing that using the Sylph scope just reveals the ghost to be pretty much a similarly ghastly creature, which funnily enough is conveniently named Ghastly.
I'll go with Missingno personally, I can just imagine this giant hyperdimensional abomination beyond human comprehension that warps the fabric of space wherever it moves and so on so forth. I know it's not a real Pokemon, but come on, it's awesome.