To start, whenever a large monster in the area sights you, a Scare Chord plays. Woe betide you if the monster is right behind you and you didn't expect it, or you're headed for an area boundary and the monster suddenly shows up right in front of you. It's a Jump Scare both out-of and In-Universe, because unless you have certain food buffs, your character flinches.
The Royal Ludroth's glowing yellow eyes underwater. Imagine that in place of Jaws...
In Tri, one of the stages of the Tundra is an ice cave. The floor of the ice cave is transparent ice, which underneath can be seen thousands of bones. These caves also tend to be the dwelling areas of the Gigginox, who in and of themselves are a nightmare.
Speaking of the Gigginox, the Khezu is horrifying. That sniffing...
To go into specifics, both are gurgling, worm like creatures that live in darkness. They both exhibit weird sounds, and they tend to appear when you're fighting other monsters. If the other entries haven't taught you to fear the dark, try seeking one out.
In Tri (but not Tri Ultimate and Portable 3rd), the caves in Tundra areas 4 and 5 are pitch-black due to a lack of light. All you can see of the Gigginox are its bioluminescent patches. Sweet dreams.
The Diablos gives a good Jump Scare whenever it erupts from the sand, and then screeches at full volume.
Any time an environment is unstable. Any of the already mentioned monsters could appear when you least wanted it, and quite a few players have been introduced to the Gigginox this way...
Rage Mode, especially for the Deviljho.
The Ceadeus and Goldbeard Ceadeus, if you think about it. The standard Ceadeus has one horn growing over its eye, which—as evidenced by the Moga earthquakes—causes it an immense amount of pain. To make matters worse, the Goldbeard Ceadeus' eyes have both fallen victim to its horns in the same way, assumedly blinding it entirely. But, like the Khezu, it can still sense where you are. But, unlike the Khezu, it doesn't need to sniff around. It just knows.
Quite a few of the Ecologies fit into this. For example...
The afore mentioned Khezu dragging a Kelbi to its doom.
The Gobul swallowing whole an entire school of fish, which simply disappear in a flash, before attacking and swallowing whole a cow-sized Epioth.
The Plesioth attacking an Aptonoth trying to get a drink, dragging it into the water with it making but a token effort to escape its doom.
The Gigginox vomiting up its young complete with high pitched gurgling. Lovely.
The Nargacuga, circa Tri, at least initially. Pitch dark forest, looking around...then suddenly, a pair of red eyes light up right in front of you. Oh shit.
One of the new monsters in Monster Hunter 4 is a giant scorpion/earwig creature, which snatches a smaller monster with no prior warning... and then fuses with it.
The Daimyo Hermitaur and the Shogun Ceanataur are based off of giant hermit crabs. So, they'd live in giant shells or rocks, right? No, they live in the massive skulls of other nightmare inducing monsters.
The Deviljho, originating from Tri, is the personification of this. They're gigantic monsters akin to a tyrannosaur, but look and act more like Godzilla in a bad mood. They're massive apex predators who can wipe out entire species when hungry, and when they get mad, they wreath themselves in their own rage (see above picture).
They have a nasty tendency to pop out when you're fighting other monsters. You could just be walking around on any assigned High Rank mission, and then BOOM. Deviljho incoming, and all you can think about is getting out of there as quickly as possible. Then there's the Savage Deviljho, providing the page image...
To further add to this, the "Everyone So Big-Big!" cinematic contains all sorts of monsters. As Cha-Cha walks along, they all sit tight (except for a Giggi)... but then he reaches the end and a Deviljho appears and starts chasing him and the hunter.
A rare occurrence is when a Deviljho moves into the area you're in JUST as you are walking into the stage boundary. You're not expecting anything and then OH MY GOD THERE'S A GIANT FREAKING DINOSAUR RIGHT IN FRONT OF ME. And of course it plays the Scare Chord of a monster noticing you. Watch out, Slender Man... there's a new Jump Scarer in town.
Remember that Rage mode mentioned above? Well, along with speed and attack boost, the Deviljho is able to hit you with defense down and severe dragonblight. Pants heavy enough?Because it gets better. That stat boost and the ability to induce dragonblight?? That's Savage Deviljho in a GOOD MOOD!
The Gigginox (mentioned above) can reproduce by itself and produce hundreds of baby Giggi in one day, take a second to think what would happen if even one days worth of baby Giggi survived into adulthood, all producing more and more baby Giggi.
One more thing that's always made the Gigginox unsettling is the fact that it's very near bilaterally symmetrical down its waist. It clearly has a front and back end, evidenced by the fact that one side dispenses toxins and the other lays eggs, but they're similar to the point that it can sometimes be hard to distinguish between the two. Of course, on top of that, it raises both ends when it roars (upon seeing you or entering rage mode). So does that mean that it...? ...Ugh.
The Giggi themselves are pretty creepy, having no eyes and a gaping mouth with More Teeth than the Osmond Family. They can latch on to you, sapping your HP until you roll them off.
The new flagship monster for Monster Hunter 4 is the Goa Magara, which is essentially a Gigginox, but worse. Take the Gigginox, paint it black, give it an insect-like exoskeleton, make it an Elder Dragon, and give it the ability to start a Zombie Apocalypse simply by having a hunter or another monster touch it or the hair and scales it leaves behind. The end result looks a lot like a Xenomorph. It's theme music isn't that bad, but its "noticed you" Scare Chord is absolutely terrifying. Oh, and the worst part? It's only in its juvenile form. There's an adult version of it later in the game, which is even even more fun.
The Yama Tsukami is also a little creepy with it looking like a floating Cthulhu head, fortunately it's peaceful in-universe except when approached by hunters.
Creepy initially; worse still when you see its mouth.
Endgame monsters like Akantor can be pretty scary the first time you fight them, with attacks that can easily OHKO you, being larger than any monster you fought up to that point, and most of the areas you fight them in, you can't get back to the camp once you enter it.
Frontier's Raviente is the largest monster in the series and possibly one of the most intimidating. When it gets hungry, it gets up and starts wiping out entire ecosystems. It takes dozens of hunters several hours to kill this thing... if they even do.
Its introductory video really shows you how hugely destructive this thing is. Depicting a peaceful island, flourishing with life, the mood suddenly gets tense and all the monsters start fleeing from something. Suddenly. the Raviente pops up from the ground, swallows a dozen tank-sized Aptonoth in one bite, then proceeds to destroy everything in its wake. A volcano erupts, the earth splits, and this is all from one monster. It really means something when everything else, up to the Espinas and the Rathian, is running away from you.