Its tongue is made of gas. If licked, its victim starts shaking constantly until death eventually comes. (Silver)
It hides in the dark, planning to take the life of the next living thing that wanders close by. (Crystal)
As if it wasn't terrifying enough that the Pokemon world was overrun with ghosts that acted like feral cats, Haunters have tongues made of gas that horribly murder anyone they touch. Oh, also it randomly will pop out of the shadows to murder literally whatever is coming by at the time. Probably by licking them with its gas tongue. Good luck ever feeling comfortable in a world where wild ghosts are committing random motiveless murders all the time.
An abandoned plush doll became this Pokémon. They are said to live in garbage dumps and wander about in search of the children that threw them away. (Emerald)
The world of Pokemon is already filled with existential dread in pretty much every corner. But did you know that even the toys in Pokemon have the potential for nightmare fuel? They do! Like some kind of terrifying mash-up of Child's Play and Toy Story 2, Banette is a doll that somehow gained consciousness purely to seek vengeance against a child. Make sure your kids are careful with their toys, otherwise those toys might come to life as Ghost-type monsters who are used to battle dragons and magnets, all while trying to avenge themselves against a negligent toddler.
Dusclops's body is completely hollow - there is nothing at all inside. It is said that its body is like a black hole. This Pokémon will absorb anything into its body, but nothing will ever come back out. (Ruby)
The concept of mummies are pretty terrifying - undead people, buried in a religious ritual, returned to the world of the living to enact some kinda curse against Brendan Fraser, etc. But leave it to Pokemon to make mummies 10x worse by having them be completely hollow within except for a black hole that sucks everything into it and - like all good black holes - never lets anything out. Honestly, I've seen way too many preschooler trainer NPCs in the Pokemon world who seem to have little or no parental guidance. In a world with black hole mummies, people should be a lot more careful.
If anyone dares to grab its hilt, it wraps a blue cloth around that person's arm and drains that person's life energy completely. (Y)
At first glance, "floating ghost sword" is a pretty cool concept for a Pokemon. I know I was onboard with it. It's eerie, but in a sorta badass way. There's nothing as inherently horrifying about it as, say, Mr. Mime. But then this comes along - the revelation that if anyone touches the sword's hilt (which everyone would try to do at some point if they were real, because - c'mon - GHOST SWORD!), Honedge will literally suck the life from them. What does a floating ghost sword need with our life energy? Some questions about ghost swords are better left unanswered.