The Grundel, possibly the most terrifying foe in the entire franchise, who is a cross between a vampire, a goblin, and a child molester. He appears to children with a mischievous nature and acts as The Corrupter, encouraging them toward increasingly evil acts and eventually turning them into more Grundels.
The really scary part? The Boogieman WASN'T a ghost. He was a demon, which meant the Ghostbusters couldn't stop him with their equipment. And Egon was very terrified of him. Which makes his eventual triumph a Moment of Awesome.
If the appearance of the Boogieman isn't scary enough, his voice when he wheezes, "I remember you" to Egon will send chills down anyone's spine, especially when they realize he terrorized Egon as a small child. Imagine being a kid and seeing that thing skulk out of your closet and speak to you in that unholy voice on a nightly basis. Eeek.
Made all that much more frightening when you realize that not only is Frank Welker using a distorted, demonic version of Megatron's voice for this thing, but it looks like a mutant monster version of the Joker.
During the climactic confrontation between Egon and the Boogieman, the Boogieman specifically opens the door to the room of the children who had contacted the Ghostbusters for help so they can watch him kill their heroes.
Watt coming close to releasing every ghost in the Containment Unit by possessing Peter. And Peter's struggle to fight from within, as control goes back and forth between them. It's one of the few times the wisecracking Peter is genuinely terrified.
Watt pretending to be a kind old lady asking for their help, specifically to get to the Containment Unit.
The house. Watt specifically designed it as a Ghostbuster trap. When Watt is captured, it collapses in on itself.
Ghash, the poltergeist in "Slimer Come Home" that would befriend the ghosts, gather them up, and eat them...and the mouth was at its stomach...
The freakiest part is after Ghash absorbs Slimer and a bunch of other ghosts, you see bulges all over Ghash's stomach as they try to get back out. The Ghostbusters save Slimer. The rest of the ghosts... aren't so lucky.
The Sandman wasn't particularly scary in-and-of himself, nor was his plan to put the world to sleep horrifying... but his horrible, horrible voice sure is.
Never mind how the... things he unleashed on NYC looked like Hieronymous Bosch had dropped acid.
The episode where the ghost-obliterating robot accidentally creates some extremely powerful ghosts from the dispersed PKE. The results were... bad.
A parody of Citizen Kane ends up being extremely creepy, with comparatively low-key hauntings plaguing the Ghostbusters all throughout.
ROSEBUD.
The ghosts in general are monstrous and particularly inhuman.
Cthulhu, now the proud page image as seen above, is extremely horrifying. This show is one of the few to be able to make Cthulhu live up to the hype of how terrifying he's supposed to be. What's scarier is that he's nearly unstoppable, even with their modern weapons.
The Ghostbusters' sinking realization after they banish Cthulhu that they still have to deal with his cult of murderous psychopaths... and how the timely arrival of the police (i.e., sheer luck) saved their lives from a very mundane threat.
The Old One in "Russian About" is another Lovecraft pastiche. The only thing Venkman can do when he sees it is scram.
Walt Flieschman's plight throughout "Who Are You Calling Two-Dimensional?" falls heavily into And I Must Scream territory. Captured and imprisoned in the cartoon world he himself created forty years ago, he’s effectively immortal. But he's constantly tortured by his most dangerous creation: Winchester Wolf. His voice actor, Don Messick, gives some genuinely agonising screams of pain. Hearing the man who famously brought sweet, cuddly characters to life begging for help is both unnerving and heartbreaking.
How do the Ghostbusters manage to escape with Flieschman? Dopey Dog and the other cartoon heroes tear the film which makes up the Cartoon World. Winchester is genuinely horrified at this and abandons his duel with Dopey to stop them. Flieschman begs his rescuers not to look back as Winchester grows to a giant size to prevent his creator from leaving. How loyal were Dopey Dog and the other heroes? They were willing to commit suicide and destroy their entire home to save their father.
The Eiffel Tower sequence in "The Ghostbusters In Paris" has an army of disturbing ghosts, camera angles designed to keep the viewer from seeing them, John Carpenter-esque music, and all the characters saying a version of "Run. Run!"
THAT. MELTING. FACE.
The ghost that feeds on electricity in "Ain't NASA-sarily so." It has many tendrils that make up the majority of its body, large yellow eyes, and large mouths with big teeth, and it's immune to the proton packs.
Egon's horrifying scream of pain when he gets hit by the de-stabilizer beam in "Egon's Ghost."
Mee-Krah in "Standing Room Only." An almost completely unstoppable Eldritch Abomination who mercilessly devours smaller ghosts (who choose imprisonment in the containment unit rather than fall victim to him), leaves deserts in his wake, and nearly smashes and melts New York to ruins, all the while emitting a monstrous growling noise. Needless to say, this was one episode that subverted the Lighter and Softer theme of the later seasons.
There's something very unsettling about seeing ghosts spooky-looking in their own right quivering with sheer terror at the thought of Mee-Krah. Worse, the Ghostbusters throw everything they've got at it and can't even slow it down until the end.
This episode's also notable for having almostnocomedic elements whatsoever: There's a bit of Cringe Comedy with Slimer trying to trick the entity by impersonating Mrs. Faversham's late father, but otherwise it's about as straight a horror story as an H. P. Lovecraft tale. (Granted, in a departure from Lovecraft, when the episode isn't being horrific, it's being heart-melting.)
Victor the Happy Ghost; as The Reveal of his true nature (he's pretending to be a cute, small, benevolent ghost but he's actually large, muscular-looking and evil with sharp teeth) is bone-chilling.
"Mrs. Rogers' Neighborhood," the episode where they visit an old lady who turns out to be a demon that possesses Peter so he'll open the containment unit.
The one where Slimer is befriended by a thing that appears in the form of a Creepy Child.
In "Ragnarok and Roll," the Anti-Villain tries to bring on the end of the world because his girlfriend dumped him. That face in the sky and the guy's nose-less face is nightmare-inducing.
How bad was it? People died during this episode - the newscaster explicitly mentions survivors during a broadcast showing volcanoes erupting, tidal waves engulfing cities, and massive fissures forming. The Ghostbusters were willing to sacrifice themselves by overloading their proton packs (which would take out everything in a quarter-mile radius) in the hope that they would be able to stop him.
The demon-possessed Ray, Janine, and Slimer's faces in "Ghostworld." Yikes. They're wearing Death Glares, Ray has fangs, and Janine and Slimer have red eyes.
Dixie from "Til Death Do Us Part" seems to be a beautiful human woman, but she isn't what she appears to be.
The creepy talking face on the door in "Knock Knock", that repeats in a low, demonic voice, "DO NOT OPEN UNTIL DOOMSDAY."
UNTIL DOOMSDAY."