If they ever decide to make a animated or live-action horror film based on characters from Pokemon, the person who made these terrifying versions of Haunter, Muk, Houndoom and Solosis should definitely be leading the project, or at least use his concept art.
Snook-8 took a handful of Pokemon, most of them not the most popular and known in mainstream, and simply took the kiddie factor out of them, turning them into the stuff nightmares are made of.
Pinsir
Haunter
Dodrio
Muk
Typhlosion
Houndoom
Cacturne
The scarecrow of the Pokemon world.
Magmar
Solosis
Hypno
Yamask
Drifblim
Gastly
Duskull
Seviper
Ghost
Cubone
Swalot
Gengar
For the complete gallery of scary looking Pokemon, check it out here. For more Pokemon alternative art, here are some awesome drawings giving Pokemon a more realistic, down to earth look.