Happy-go-lucky Chopper on the 3rd Rumble Ball. He turns into a giant mindless, hulking killing machine.
What got me about that was Chopper's last conscious thought before he went berserk. "Please don't let anyone enter this room." He knew what was about to happen.
However the ONLY ones who dare stand up against the Celestial Dragons were Monkey D. Luffy and Fisher Tiger.
Slavery. Slaves are forced to do all their master's deeds until they die of exhaustion. If they try to escape, their collars explode. No matter how bad they suffer, they are forced to smile, to show respect to their abusive masters. In fact, slaves don't even have the right to cry! Even though there is no romance or sexual intercourse in this manga, it is very likely that female slaves are sexually abused (which explains Hancock and her sisters' hatred of men).
In Hancock and her sisters' case, they were even fed Devil Fruits to satisfy their masters. Sexual undertones aside, it's still fucking scary, because it definitely means their masters enjoyed watching people suffer.
Caesar Clown: the hellish child of The Joker and a Hannya mask, expert of Weapons of Mass Destruction, capable of turning his body into a mass of deadly gas and using his own men as guinea pigs to test his biological weapon all while he plays the good guy's role to them. Now that you know this, think again about all those kidnapped children and let the Fridge Horror in.
It gets worse. It's revealed that Caesar was drugging those children by slipping a stimulant drug known as NHC10 on the candies fed to them, to the point where depriving them of it generates not so pleasant◊results◊. Not only does it cause intense pain (The kids were even writhing in agony !), but it also causes the person to go absolutely apeshit, and start attacking everyone at random. Seeing those cute children who begged the Straw Hats to save them suddenly become like drug-craving, bloodthirsty zombies (Complete with Blank White Eyes) is extremely creepy.
Oh, and if you happen to have some kind of immunity to his poison gas form? He can just remove the oxygen from the air around you so you suffocate. Sweet dreams!
In general, the lengths to which the World Government is ready to go to "uphold peace" are frightening. For example, when they learned that Gol D. Roger had a child, they didn't know who the mother was. Their solution was to execute any infant, including unborn ones, that could be in any way related to Roger. That's bad enough, but even their mothers and any one related to them, were arrested and killed. Not to mention they don't even seem to care about upholding peace that much anyway. After all, not only did they cover the escape of many insanely dangerous Level 6 prisoners from Impel Down following Blackbeard's breakout just so they could save face, but they also let the World Nobles (See above) do as they please.
Buster Call. With the push of a single botton, entire islands are erased. To give you an example of how nightmerish the Buster Call is, Robin, who not only maneged to escape her islands' demise at the hands of the Buster Call, but is also one of the most fearless people in the entire story, is completely terrified of the mere possibility of a Buster Call being unleashed.
We also has the Thriller Bark arc, which featured a gigantic ship shaped like a Haunted Castle that was home to Dr. Hogback, a Mad Scientist who made zombies (including zombiefied, literal Mix-and-Match Critters), Perona, an Elegant Gothic Lolita girl who could summon ghosts that came out of the goddamned walls (and floors and paintings), and Big Bad Gecko Moria, a large, vampire-like man who could steal people's shadows, and briefly traps Luffy's shadow in the body of a humongous zombified giant.
The wacky personalities of the various zombies, including Oars (zombified super-giant) act as pretty effective Nightmare Retardant. Because its hard to be terrified of such very silly creatures... Though the method of creating them and the effect of the sun on shadowless people is very Nightmare Fuel-worthy.
The most nightmare-inducing part of the entire arc is, without a doubt, Moria's Villainous Breakdown. His greatest zombie defeated, Moria snaps, ranting about his Freudian Excuse, before entering his One-Winged Angel form in what might be the most terrifying transformation sequence out of this show. And while he's ranting, he's got a most terrifying◊ Slasher Smile.
Even after all the Zombies the scares aren't over. Enter Bartholomew Kuma. Bible toting titan of the Warlords of the Sea. With the other Straw Hats broken and beaten, he was going to perform clean up. Zoro manages to plead that he be taken instead of Luffy. Kuma agrees, but not before laying down the line to just the right music.
Kuma: But before you can take his place, I will make you experience Hell.
He wasn't exagerating. He extracts all the pain and fatigue from Luffy, which is manifested as a red bubble as big as Kuma, and puts it into Zoro. To elaborate, when one tiny little baseball sized blob of the pain bubble was pushed into Zoro, he reacted in sheer agony as if a bomb just went off in his chest, coughing up blood.
The Blugoris from Impel Down are freaky. Large, vaguely humanoid creatures wielding giant battle-axes... with no face. Just a small lump for a head and what appears to be a painted-on skull.
Impel Down itself. All prisoners are first boiled alive upon being sent there, and part of the punishments involve walking upon a field of razor sharp blades until your feet are nothing more than shreds. The kicker is that this occurs on the first level. The tortures get progressively worse as you continue further deep into what other characters in the manga have explicitly referred to as 'hell on earth'.
And then we get the oh so lovely image of Luffy covered in poisoned goop, with face frozen in an expression of agony and horror, like he's screaming. Sweet dreams.
As if that isn't bad enough, Ivankov's treatment for it is even worse. Imagine everyone's favorite doofy pirate captain screaming nonstop in agony for two straight days as his body breaks down and regenerates. The fact that he got over it in less than a day is definitely a mercy.
Heck, Akainu in general, especially after he recovers from Whitebeard's beatdown and proceeds to chase after Luffy. The man's a damn tank and nothing is going to stand in his way. Not even the toughest pirates of the One Piece world.
Even scarier : from the same arc, we have the anime version of Luffy's breakdown.
Also in the Marineford arc, in the anime, we're treated to a shot of three Pacifista slowly walking towards the camera, with their faces silhouetted, aside from their glowing eyes. In the background we've got what looks like a large fire illuminating the Pacifista. It's pretty unnerving.
Rewind back into Alabasta, and we all remember the lovely scene of Crocodile sucking all the mostiure out of Mr. 3. Seeing a human whither into a mummy, as Mr. 0 puts it, is very uncomfortable. Made all the worse when he does the same thing to Luffy.
Not to mention Crocodile's systematic breaking of Vivi and the berserker frenzy he whips the rebels and Alabastan army into, to the point that even after everyone is knocked over by a gigantic explosion that would've wiped out everybody if not for Pell's Heroic Sacrifice, they just get right back up with crazed expressions and go right back to fighting.
Post Time-Skip
Caribou. Just Caribou. He is a psychotic homicidal lunatic who prays to God to forgive his victims of their sins.
His swamp Logia powers are really grotesque looking, too. He looks like a puddle of melted flesh, although this can be attributed to being black and white and Oda's art style.
Also the way he was kidnapping those mermaids eerily resembled a rapist assaulting his victim.
Hody Jones' latest form can be this, with that huge, gaping murloc-like jaw.
Hody Jones in general. He's basically One Piece's response to Adolf Hitler...or at least, Hitler if he had been stopped before he could have achieved his dreams of conquering his own country...
The anime just reached the original post. Pleasant dreams...
If it's any consolation, he doesn't get a chance to do anything other than receive a truly incredible No-Holds-Barred Beatdown from Luffy, which he definitely had coming.
And now we have a look of her in the Anime, complete with blood-red eyes just like Hody, which somehow manages to make her even more terrifying.
The guys in Chapter 656 who kidnapped the remaining Straw Hats look pretty fucking creepy with those diving suits.
Wait, there's more: brought to an island half on fire, half frozen, in a nursery-like room full of giant children. Oh, and what exactly "divided" that samurai like that?
Recent chapters suggest that the newest member of the Seven Warlords of the Sea, Trafalgar Law, did it to him.
Two words: frozen corpses.
We find out that Trafalgar Law was able to join the Seven Warlords of the Sea by sending 100 pirates hearts to the government. Knowing what the government would and could do with those hearts, what do you think has happened to those pirates?
Him doing it to the Straw Hats, is actually presented as a comic moment, since it results in a Freaky Friday Flip. But then he pulls off a different procedure to Smoker; we see his actual heart. Not so funny.
A bit which could also fit into Fridge Horror is that (be careful, triple spoiler ahead) the 100 pirates who got their hearts stolen by Law are definitely not dead, as Tashigi was perfectly OK in Smoker's body despite the fact that Law had stolen Smoker's heart.
Speaking of Fridge Horror, Caesar Clown is an "insane bastard" (Smoker's words) whose primary field of expertise as a Mad Scientist is mass-murder weapons. And he holds kids captive inside his main facility. Just what the fuck does he intend to do with them ?!
The Blob Monster Slime released by Clown in chapter 668 isn't a pleasant sight.
It's not just its looks that are monstrous, it's also basically invincible. It constantly releases poisonous gas, so you can't get too close without a mask, and cutting or shooting it just makes it spew out more. Plus, if you touch it, you are instantly infected with its poison, and the effects aren't pretty. So, if you can't attack it with weapons, or even attack at all, what's left? How about fire? Nearly all creatures fear fire, so let's try torching this one. But wait, this thing's made up of gas, so it'll just explode if set aflame. And even though it's not good with water, the poison that it spills in still works. And then it shows off its Devil Fruit ability.
And things just got even worse: after eating a bait made by Caesar, it changed into a huge cloud of gas that petrifies its victims fast as shit. Bonus points for how scary the statues look and the guy begging for help as he's being petrified.
AND IT GETS WORSE.... Caesar mentions that apart from the petrification, it's the same gas as before; that is, it cripples, but the victims live long enough to crawl away when they had no legs. Fridge horror ensues since the gas must take hours to actually be deadly, but now paralyses instantly, effectively meaning that the victims merely die of suffocation a minute or so after getting paralysed. Of course, if the ash is porous, which may well be the case considering it's supposed to be an improvement of the same gas weapon, then they simply stay petrified for a long time becoming increasingly poisoned before it eventually kills them. So the only immediate effect is to petrify alive.
Monet's monster form is actually pretty frightening. Seeing the Cute Monster Girl grow a giant set of teeth capable of biting off a Marine's shoulder.Brrr.
Later on, Zoro cuts her in half vertically. She survives, but that was only because Zoro decided not use Haki. Because Monet is too scared of what Zoro could have done to her, she is unable to put her body back together right away. Seeing her two halves moving about erratically, melting into snow and getting back into shape time and time again, is disturbing as hell. The look of absolute rage on her face(s) is not a pretty sight either, and gets even scarier when you compare it to how calm and cute she looked before.
Movies
Movie 6. Plant-men suddenly wilting into dead husks, the Straw Hats being absorbed by a giant plant monster while the villain announces their death, the image of Luffy made into a human pincushion trudging zombie-like while begging for his crew to be returned... it's no wonder the movie is controversial among the fanbase.
One of the worst parts of the absorbing sequence was when Zoro is the last one absorbed. Mainly because, while the others appeared to be unconscious, Zoro has this look on his face that makes one think he's completely aware of what's happening. His mouth is open like he's screaming! What about toward the end when the flower on the villain's shoulder turns into this giant, grotesque beanstalk-thing, and you can see the bodies of Luffy's crew jammed in it?
The cutesy Lily avatar on the Baron's shoulder cheerfully exclaiming that it's hungry, and making biting and chewing motions both during and after absorbing the Straw Hats.
Just to add a new dimension, Director Mamoru Hosoda has said in an interview that the movie is an allegory for his work on Studio Ghibli's Howl's Moving Castle.
