These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Nightmare Fuel: The Weedle spawning grounds in Viridian Forest, the monstrosities in Mt. Moon, the creatures in the fog of Lavender Town, the scenes inside the Pokemon Tower, how Haunter attacks Team Rocket's Charizard... let's face it, this entire Fic is Nightmare Fuel.
Haunter is becoming more and more of one. If it's not reducing people to bloody messes, it's laughing maniacally about doing so.
It gets worse when Mewtwo visits Ash and directs him to rescue Sabrina from Team Rocket. Haunter's behavior upon sensing Mewtwo's mind reaching out to them suggests that Mewtwo is strong enough to scare even Haunter, despite the fact that Mewtwo would suffer from an Elemental Rock-Paper-Scissors related disadvantage. The disadvantage had been previously stated to render psychics almost helpless against ghosts: the implication is that Mewtwo is a powerful enough psychic that he can still easily take down ghost types.