A quiet, understated moment of heart-crushing power in the aftermath of Godzilla's attack: A doctor passes a Geiger counter over a little girl. The counter goes berserk.
The Oxygen Destroyer is one of the most terrifying weapons in fiction. Sure, it's not as awesome as a laser cannon or as stylish as a light sabre. But, the fact that the Oxygen Destroyer causes its victims to have a horrible agonizing death via asphyxiation is nothing short of terrifying.
It doesn't just asphyxiate, it dissolves living things all the way down to the skeleton!
Godzilla himself is rather terrifying. This isn't some random confused animal in the wrong place at the wrong time. No, he knows exactly where he is. Oh, and he's pissed at humanity and wants to kill everyone. Yes, even you. Worse yet, there's almost nothing you can do to stop him. Something that's big, radioactive, and angry is not something to be taken lightly.
The GMK incarnation of Godzilla. He's essentially a giant zombie version of the original Godzilla controlled by the vengeful spirits of forgotten soldiers who died in WWII. White souless eyes, a permanent scowl on his face, wants to wipe out all of Japan (and possibly humanity in general), and even ends up killing three gods (IE: Baragon, Mothra, and Ghidorah) that were sent to save the world from him.
And, at the end of the film his disembodied heart is still beating at the bottom of the ocean. In other words, you can't even kill him meaning he'll soon regenerate and come back to wreak havoc once again.
Destroyah. To quote James Rolfe-"He looks like the goddamn devil!" It doesn't help that his juvenile forms look like a cross between a Xenomorph, a crab, and a scorpion, or that he's able to use the same horrific ability as the Oxygen Destroyer (see above, also where he got his name).
Oh and the comic version of him doesn't help either. When he's fighting Anguirus, he tries to pry his jaws open.
There's a scene in Godzilla VS Hedorah where Hedorah is flying overhead emitting a cloud of sulfuric acid. The innocent bystanders below are shown collapsing and then their flesh melting off of their faces.
Speaking of that scene, Riichiro Manabe's score during it. Say what you want about his composing ability, but when it comes to producing horrific cues for this and other films such as Crest Of The Wolf, the Bloodthirsty trilogy, and Jigoku he's actually pretty good.
Hedorah tries to drown Godzilla in sludge. Godzilla's struggles to get out make it all the more unnerving.
One scene in the movie has a character hallucinating that everyone at a dance club has a creepy fish head. It's one of the most disturbing Big Lipped Alligator Moments in any film.
King Ghidorah, even with his Badass Decay, falls under this. Try to imagine a three-headed dragon that travels from planet to planet essentially wiping out all life from it. Oh, and that he's often under the control (or at least a willing servant) of various alien races that want nothing more to take over the planet. Don't think it can get worse? Well, take in the fact that he ENJOYS his job.
The Heisei incarnation's origin is rather terrifying. It pretty much starts out as three cute genetically-engineered pets that are left alone to fend for themselves on an island near Japan. Then the island is hit by a nuclear bomb (the same one that created the original Godzilla) that causes said three pets to fuse with one another into a single giant dragon.
Many people think that King Ghidorah's high-pitched roar is Narm...but listen more carefully, and you'll notice it sounds like psychotic giggling.
The GMK incarnation of Ghidorah seems to be horribly weak at first, until you realise that's just because he's a juvenile and hasn't grown back all of his heads yet. Oh, did we mention all of his heads? Well, that's because he happens to be the Yamato No Orochi. No wonder his soul was purified prior to the events of the film.
Ghidorah gets his own pretty awesome invocation of nightmare fuel in IDW's Half Century War miniseries by James Stokoe; After Space-Godzilla and other in-continuity monsters are taken down, Ghidorah and Gigan are shown in silhouette at the end of the fourth issue en route to Earth. The fifth is entirely devoted to the human lead and Godzilla taking them down, suggesting those two are the very worst the universe can throw at Earth.
Gigan. Sure, he's not the scariest-looking Kaiju out there. But, take into consideration that he's the first monster to make Godzilla bleed. Oh, and there's the little fact that he absolutely loves causing mayhem and destruction and prefers to hack his opponents with either his scythe-arms or his buzzsaw-belly rather than use his optic blasts.
Biollante, Anti-Villain and Dark Is Not Evil applying her second form is still really scary to look at. In spite of being the monster to root for it's disturbing when she pierces Godzilla's hand with a vine and then easily fits his whole head into her mouth and snarling. Less than five minutes of footage and no one forgets that monster.
One of the scariest (and saddest) aspects of Biollante is her Roseform's cry. It sounds like a human girl wailing in fear. Erika knows what she's become, and she's scared.
Then there's the fact her mouth in her second form looks like a crocodile head and has MULTIPLE ROWS OF TEETH even on the roof of her mouth and on her tongue. Not surprising seeing as the person who came up with her was a dentist.
Monster X. It's pretty much the only opponent to hold the big G up (apart from Caesar, Anguirus and Rodan teaming up) and its roar/Evil Laugh is scary as hell. And guess what? It's Kaiser Ghidorah, and he's even worse.
The Godzillaverse itself qualifies! Imagine living in a Crapsack World choke-full of Kaiju, aka humongous, near-indestructible and VERY pissed-off monsters who gleefully screw the square-cube law(aka something so big should not exist, yet it does). they include mutants, ancient deities and alien planet-destroyers. at any moment they may attack your city and raze it to the ground, millions of people will die, there is no time to rebuild because probably they have already spread death and destruction somewhere else, humanity can't do anything to stop them. the kicker? the most heroic of them (Godzilla himself) is a dinosaur mutated into a walking nuclear holocaust, and he is one of the most destructive. Sure, he's more of a Chaotic NeutralAnti-Hero, but he still doesn't give a shit about mankind.
For some, Godzilla's iconic roar has a hell of a lot of scare potential. No living thing should ever be able to produce that kind of eerie, high-pitched wail. It makes him sound less like a roaring dinosaur and more like some kind of ethereal demon—which actually makes sense, when you consider that a life-form that big isn't scientifically possible in Real Life.
This is intentional. Toho Studios couldn't find a suitable animal-like roar that would fit a creature Godzilla's size. So, composer Akira Ifukube created the trademark roar by running a leather glove over a double bass. It's changed slightly over the years, but the general "SKREEEOOOOONK" aspect has pretty much stayed the same.
The Half-Century War invokes this with Godzilla's first appearance. There isn't even an attempt at onomatopoeia; just jagged, broken sound waves in a speech bubble. You know what it sounds like.
The sea louse from The Return of Godzilla has a longer, overdrawn and almost humorous intro in the original version. The American cut makes the scene shorter and actually terrifying.
In the Dark Horse Comics series of Godzilla, G-Fans were given plenty of scary moments. What really stuck out was the arc when Godzilla got poisoned and started to bleed from his mouth and his eyes. It doesn't really help when he's acting like a rabid animal... a giant rabid animal that breathes fire.
The first Dark Horse Comics Godzilla comic. All of it. The creepy atmosphere, Godzilla's design and that worm-''thing'' that was sleeping in the slab that summoned Godzilla in the first place.
Burtannus. Although she is not what you'd call scary, she has her moments. The first is when she is attacking a village in Brazil and she kills someone with her lightning. You can actually see the guy's skeleton! Also, her first appearance in the comic when Burton Helzer and Kino come across her young.
Bagorah. Just Bagorah. From his design, his method of attack and his insatiable appetite for flesh. He even tries to make a meal out of Godzilla.
Mechagodzilla's first appearance was pretty frightening as well. One of the first monsters he goes after is Godzilla's longtime sidekick, Anguirus. At first, it just looks like the Godzilla we all know and love has snapped and is on a destructive rampage. Anguirus tries to stop him, only to get the stuffing beaten out of him. Then "Godzilla" reaches down, grabs Anguirus' jaws and breaks them. He more or less breaks his lower jaw in half. What makes this even more disturbing is that, at this point, Anguirus has no idea this isn't the real Godzilla, his friend doing this to him.
Anguirus knew that it wasn't Godzilla, that's why he attacked it.
The ending of the ongoing IDW Godzilla comic issue 11 which has Godzilla going up against Space-Godzilla and Monster X who, up to this point of the comic, have proven to be some of the most powerful monsters so far.
Issue 12 gives us a multitude of Nightmare Fuel. The remaining Monster Kill Crew trying not to drown while Mecha-Godzilla is being flooded in the midst of a two-on-one monster fight, Monster X giving Godzilla one brutal smack down, even getting him to bleed and finally transforming into Keizer Ghidorah which even makes Godzilla have an Oh, Crap moment.
Issue 13, the fight between Godzilla and Keizer Ghidorah.
Godzilla: Kingdom of Monsters has a ton of this. The first issue has Godzilla kill two kids just by rising up from the ground. And it gets worse. We are then introduced to the evil psychic twins who manage to gain control over Rodan and Battra and hope to take control of Godzilla later on in the series, lots of unsettling apocalyptic imagery and the undeniably brutal monster fights. But the creepiest and scariest moment comes in issue 5 where there is an exodus of people evacuating Los Angeles due to Godzilla's attack and, while a fight breaks out on the road, Godzilla shows up and outright vaporizes everyone on the road except for a little girl who ends up traumatized. In fact, the aftermath looks just like a nuclear bomb hit the area. Then there's the animal deaths which precede a monster showing up.
In Rebirth of Mothra, we are given Desghidorah who looks less like an Expy of King Ghidorah and more like a three-headed gothic dragon who can split the ground open and unleash flames from it. In its sequel we have Dagarla who can basically create a waterspout and he releases poisonous starfish... THINGS. Then we have its own take on King Ghidorah who has his Eldritch Abomination status beefed Up to Eleven as a soul-eating monster who can regenerate from a single surviving piece of himself. Also, his first target when he arrives on Earth are kids who he puts into a dome to save for a snack. Not only that, how he teleports them is disturbing, especially when he rips the daughter of the main character's parents right out of her father's arms.
In Rulers of Earth, just before Zilla makes landfall, we briefly see his eyes in a tidal wave with someone surfing in it. Chris Mowry and Matt Frank have officially made Zilla scary. Well, maybe not scary but unnerving. Once on land, he tries to eat the main character.
Godzilla: Rulers of Earth issue 3. Manda destroying the USS Goldenstein while Gezora finishes off the rest, the appearance of the Devonians and Cryog and a juvenile Destoroyah infiltrating a convention and killing people. Then there's the ending which has Godzilla arriving on Malibu beach with a giant scar in his chest before he collapses. Prior to this, Godzilla was dragged under the water by some unknown Kaiju.
The next issue reveals the creature who majorly wounded Godzilla was Destoroyah.
The latest arc from the Rulers of Earth series has so far started off with the return of Minette and Mallorie as well as an introduction to a mass-produced line of MechaGodzillas, one of which manages to nearly kill Anguirus.
As if that wasn't enough, an entire army of them is deployed against Godzilla.
How Mecha-King Ghidorah was established in this universe. Basically, the Cryog found King Ghidorah and used Gigan to lop off his middle head and replace it with a robotic one and, considering the other two heads' reactions, it was rather painful.
The fate of Minette and Mallorie. Basically, they end up seemingly lobotomized.
The teaser trailer for the 2014 remake manages to use a J. Robert Oppenheimer quote to frightening effect.
"We knew the world would not be the same. Few people laughed. Few people cried. Most people were silent. I remembered the line from the Hindu scripture, the Bhagavad Gita. Vishnu is trying to persuade the Prince that he should do his duty and, to impress him, takes on his multi-armed form and says 'Now I am become Death, the destroyer of worlds'. I suppose we all thought that, one way or another."
The Mutos also seem to be a highly-adaptable and dangerous species, and Godzilla is larger than the 100-meter Godzilla. One of the teasers reveals that one of the Mutos are barely up to Godzilla's knee in height.
The miniseries Cataclysm finally introduces Kamacarus into the IDW comics with their Final Wars design but a slight tweak to their mouths. They are terrifying as they viciously hunt down and pick off a group of human scavengers.
In fact, the entire story of Cataclysm is terrifying. It's set several years after an unknown event caused multiple Kaiju to attack cities all over the world before disappearing for some reason, leaving human civilization in utter shambles and the Kaiju being seen as nothing more than ancient gods whose wrath was justifiable punishment towards mankind or scary stories told around the campfire... only for them to come back.
More out of universe but ever wondered how they created the silk spray for Mothra larva and Kumonga? Molten styrofoam. And they would spray this non stop at people in big, if thick, heavy suits.
The very first film makes great use of the Nothing Is Scarier and Hell Is That Noise tropes... we're not even introduced yet to Godzilla, yet we can already hear his thunderous stomping and roaring at the very beginning all while the movie's main theme kicks in with full force, creating a truly nightmarish effect.