Wario's "crushed flat" and "turned into a spring" transformations in Wario Land II and 3 are pretty horrifying, especially when you first see them. It's one thing when that stuff happens to cartoon characters — quite another when it happens to "you" in a game.
Bobo's design in the first game, especially if you've played II first, where his design is very toned-down. To illustrate: This◊ is his design for II, while this◊ is his original design.
Rice Beach has a scrolling stage where you must avoid bats trying to knock you off a tiny platform while a huge spike-covered enemy chases you. If you fall off or touch the spiky enemy, you instantly die. It's very, very nerve-wracking.
Earn all the treasures and complete the map in Wario Land II (that's a 100% Completion) to unlock The Really Final Chapter. Its only level is this... monstro◊sity◊. The enemies are petrified inside bubble gum with disembodied, realistic eyes, ears and mouths everywhere. The music does not help one bit. Rated E for Everyone.
Worth noting that this level directly inspired both the music for Forest of Illusion and the design for The Warped Void, both from 3. While greatly toned down, both elements are still trippy and unsettling.
Let's start out by saying that in Wario Land 3, nothing can kill you. Being hit by enemy attacks causes some kind of inconvenience, such as knockback or a status effect (being set on fire, for example, causes Wario to run uncontrollably until he touches water or until the fire envelops him, giving you a short time to walk around, invincible and able to use the fire to solve certain puzzles, at least until it reduces him to a pile of cinders. Which he walks out of completely unharmed, being indestructible). After retrieving the Plot Coupons, you return to the temple holding the being that has advised you over the course of your quest to find out that, on top of simultaneously being the Treacherous Advisor, the Big Bad, and a Monster Clown, he is the only thing in the game that can kill Wario. Seeing the Game Over screen after a whole game without it can render some kids unable to finish the game.
And just to rub it in, the game saves right before the Game Over. Final Death paranoia, anyone?
That ghost thing in The Vast Plain. It does not look harmful, but if you get close enough to it, it actually zooms down on top of you and makes the screen around Wario and the ghost all black, all the while closed-eye grinning. And the only way to reverse it is to bump into a wall or something similar.
The giant snakes in some of the levels. After the snakes rise from their urns, their heads turn red, make a cacophonic hissing and spew fire from their mouths.
The Robo-Mouse guard in The Stagnant Swamp and The Peaceful Village. Think of it as a less deadly version of the Ultimate Chimera from Mother 3 but don't think that makes it any less frighting. Sure the thing doesn't kill you and doesn't appear in many levels but if you touch it at all its alarm goes off and it dumps you outside, and another comparison to Mother 3's Ultimate Chimera is that the thing can't be killed, and it's a hell of a lot startling when the thing catches you and scares the hell out of you when you've tried to avoid it when and/or you get caught by it and haven't expected it especially if you thought you were safe.
The Golden Diva from Wario Land 4 falls straight down, right to the bottom of the Uncanny Valley.
And the fact that she absorbs the black cat (who is actually Princess Shokora) when it attempts to oppose her.
Not to mention there's a Scare Chord as soon as you enter her room (If you don't use items that is) with her nowhere to be seen, the room is very dark, there's this mist with her slowly floating down with haunting music with her creepy laugh
Her boss music's creepy as well. It starts off slow and ominous giving this creepy feel and then it eventually goes into a more disturbing version of the Hurry Up theme.
Some of the enemies in Wario Land 4 are pretty disturbing. In particular, several enemies in an early lake level tend to have huge fangs suddenly appear on their mouths as they try to bite you.
In general, most of the audio and sound effects in Wario Land 4 are incredibly bizarre at times. Then, there's the "HURRY UP" tune.
It's worse when you do run out of time, there's this frantic beeping as Wario runs out of coins and the backgrounds turn grey, once you run out of coins, Wario gets kicked out of the stage while a creepy clock echos even when you see Wario just landing outside.
Spoiled Rotten takes the cake. Once you get her health down to a certain point, she suddenly gets a disturbingly detailed face and huge, razor-sharp teeth.
This is one of the more overlooked moments, but in the instruction booklet, Wario says that there's a rumor that no one who's entered the Golden Passage has come out alive, leaving the comment "Scary!" It's quite a creepy foreshadowing considering what the Golden Passage is like.
Chortlebot in general. It's a flying, robotic clown head that tries to kill Wario with a flamethrower, and laughs maniacally while doing so! And it can be a pretty tough boss too, if you don't know what you're doing.
Bad Manor and Boogie Mansion. The former is your stereotypical haunted mansion, but the latter turns the background blood-red and adds a new undead enemy that can rebuild it. It can make completing the game much more scarier as it is the final level in the game.
The last few seconds on the timer has the music switch to the Shake King's theme. If you do not make it, your money is shaken out of you and you're thrown to either the beginning or the checkpoint.