One early episode featured a monster called the Terror Toad with a glowing energy tongue that would wrap around a Ranger and lick his or her head, turning the Ranger into a ball of energy that would go into his mouth, and a cartoonish imprint of the Ranger's face mask could be seen on his stomach, on his first encounter with the Rangers; in the following one, he switched to a solid tongue that bodily pulled the Ranger inside. It didn't help that this monster managed to eat all but one ranger (Prior to Tommy joining the team) and left Kimberly all but powerless to stop him from eating her as well.
The Invenusable Fly Trap monster that ate the Power Rangers was made worse by its voice: Specifically, its usual voice was very high-pitched and reminiscent of an old lady, but it could switch to a deeper, demonic-sounding one. When it tells Heel Face Turner Tommy that it'll give the others back if Tommy rejoins the villains (and this is after it ate the other Rangers), after Tommy refuses, it says "(high voice) Fine with me! I was (switches to deep, demonic voice) very hungry (instant switch back to high voice) anyway! (Evil Laugh)"
The Rangers fight Goldar and Scorpina. After a bit of swordfighting, Rita does the whole magic-wand-grow-Godzilla-size shebang. While Goldar kept his form, Scorpina mutated into something that looked like a hybrid of the Predator, a cobra, and a scorpion, complete with dreadlocks and a fleshy whip for one hand.
It's made even more jarring by the fact that as a human, Scorpina's hauntingly beautiful. Also, her Prehensile Hair ends in a sharp hook that can latch onto an opponent and administer Electric Torture.
Evil!Tommy. Yes, there is some Narm at play here, but seeing Tommy, all glowing eyes and Evil Laugh, can be slightly off putting if you're accustomed to his more familiar characterization. Credit to Jason David Frank, he plays T.O. with enough underlying malice that if you didn't remember what show you were watching, you'd swear he was about to snap necks on a dime.
An early Wham Episode featured the hitherto undefeated Megazord being completely and utterly trounced by the giant forms of Goldar, Scorpina and Tommy, who was still evil at the time. Moreover, a solar eclipse rendered the (solar-powered) Megazord unable to defend itself to the best of its ability. The villains suddenly foregoing the Monster of the Week premise and fighting three on one is enough of a shock as is, even if it didn't show each of the Zords sinking into lava with agonized metallic screeches.
Rita has a special advantage most other villains don't: with her telescope and witchcraft, she is constantly aware of where the Rangers are. That means most of her attacks come in the form of sneak attacks, aiming to pick off the Rangers in compromised positions. Imagine the psychological stress that must cause the Rangers, knowing that Rita could attack at absolutely any moment. The one safe place on Earth is the Command Center, but even that is shown to be vulnerable in a few episodes.
The opening of "Calamity Kimberly" shows Squatt spying on Kimberly from her bedroom window and "For Whom the Bell Trolls" has him sneaking into Trini's bedroom while she's sleeping to transform her Mr. Ticklesneezer doll into the Monster of the Week right there and then. They teleport away instead of attacking her while she's at her most vulnerable with her guard down, but still. To younger viewers who may already struggle with nightmarish ideas such as monsters under the bed, seeing Squatt spying and entering into the protagonists' bedroom while they sleep certainly won't help them do that at night.
"No Clowning Around" has the story taking place in an Amusement Park of Doom where Putties and the Monster of the Week Pineoctopus are disguised as Monster Clowns, Pineoctopus as "Pineapple the Clown", who subjects poor Trini to a large helping of Mundanger fright when she notices him luring her little cousin Sylvia away when the group is distracted and has to frantically look for them in the crowd. Then he transforms her into a cardboard cutout ramping up Trini's horror until Alpha figures out how to revert her back to normal later on. When the Rangers fight the Putties, Pineapple manages to lure Billy away from the others and eerily comes up from behind him with a Slasher Smile to turn him into cardboard, but Billy sees him in a reflection and ducks down letting a Putty Patroller get transformed instead.
Goldar's Dark Dimension in "Green With Evil": a cage area, with the floor filled with a thick mist, with nothing else there! Outside the cage is an empty space that goes on forever, and inside there's Goldar, hunting down Jason, who is unmorphed. Jason is hiding in the mist along the floor as Goldar stabs around with his sword, nearly impaling Jason's head at least once.
"Green With Evil 5": The Rangers had discovered who the Green Ranger was, and Kimberly went looking for him to futily try and talk him down. Finding Evil!Tommy at the Youth Center, she tells him they know... and he hits a Death Glare that visually terrifies Kimberly, before warning her of the Rangers' impeding demise. Again, if you only know Tommy as The Hero, you'll be unsettled by this whole scene. Credit to Amy Jo Johnson, her wide-eyed fear was utterly palpable, as she knows Evil!Tommy could've killed her right then and there, Rita's orders being the likely reason he didn't.
All the Rangers except Kimberly becoming Brainwashed and Crazy thanks to the Dramole in part two of "Return of An Old Friend", especially the bizarre way one of them yells "Attack the Pink Ranger!" in a slurred, almost zombie-like voice.
The piranha monster from the Season 2 premiere, Pirantis Head, from the maker of "Invenusible Fly Trap."
The Season 1 monster Goo Fish (not one of Zedd's, though they share a voice actor) had a fearsome face, and after taking enough attacks started to come apart messily.
The Lizzinator. Even the goofy "Ah-nold" voice doesn't quite gloss over the fact that he's pretty much INDESTRUCTIBLE, to the point that the Rangers have to summon a special Zord that's basically a giant cannon on wheels to take him down. But not before he beats Jason half to death behind the school in the middle of the day, while everyone is still attending class.
Rita's War Zord Cyclopsis, from "Doomsday". Rita brings her palace to Earth, giving her the power she needs to raise it from the mantle. It was scary enough to make the Megazord's knees tremble. It took the Ultrazord to beat it.
Cyclopsis hacking parts of the Megazord and Dragonzord off definitely counts.
The scariest part is when Rita causes Titanus to sink into the earth. Titanus is the strongest of the Zords, unstoppable. Nothing stands up to the Ultrazord, which is pretty much "the other Zords ride its back." While it doesn't have much in the way of facial expressions, somehow those limited movements, combined with its roar and eye-lights, convey the pure terror that the invincible machine is in as the ground devours it with only a gesture from Rita.
And then there's Lokar. A giant floating blue head with spines all over it that Rita can summon. Nothing is mentioned about his background (other than that he almost wiped them out the last time Rita summoned him). He has breath that can enhance monsters (as he did to Mutitus), shoot lightning, and causes general destruction. Only the Ultrazord can make him flee the first time. The second time, he assumes a new, scarred form (presumably thanks to the Ultrazord's barrage) and helps bring back Cyclopsis. This time, he isn't shown retreating; he just disappears (they cut the sentai shot of his destruction, since it was simultaneously disturbingandcheap). His Zyuranger counterpart was also Satan. (Additional cut shots included his mutation of Mutitis into his final form — Mutitus' skull split open.)
"Putty On the Brain" sees Zedd curse Zack and Billy to see the other Rangers as Putties. Note, this was when the Z-Putties were still presented as Elite Mooks and the episode goes out of its way to convey how frightening and unsettling it is to see your enemy's soldiers all around you, even once you realize they're your friends. Worse yet is how when fighting real Putties, they can't tell friend from foe and nearly attack Trini.
Eye Guy was a Season 1 monster whose body is comprised entirely of eye balls.
Similarly, Season 3's See Monster looked like a old bald man wearing a flasher's trench coat... that would reveal fleshy innards with eye balls sticking out all over whenever it was open.
Pumpkin Rapper has the charming ability to encase the Rangers' heads in pumpkins, leaving them unable to breathe.
Lord Zedd himself is this trope personified; a skinned corpse with metal armor/featureless faceplate over his crotch/face and tubes that pumped blood throughout his body that was portrayed as competently evil with the Evil Plan victories to prove it, to the point that he was toned down to reduce the scariness factor.
Honestly, if you just saw a picture of Zedd and didn't already know he was from Power Rangers, it wouldn't be unreasonable to assume he was the villain of a horror movie, a la Pinhead.
And while he became less scary in demeanor due to said complaints, Season 3 gave him way more successes than ever before, bringing way more Darkest Hour moments in a single season than most of his successors, including nearly killing Kimberly, thus still keeping him scary in another way. Honestly, starting from Changing of the Zords, he screwed them over so much that the Rangers were pretty much on plain defense from that point on, even with all the minor recoveries and victories they were still claiming, up to and including destroying the Ninja Coins and blowing up the Command Center, which ends the season. In the larger scheme of things, his reduced scariness doesn't take away that he became scarier in competence, with only the arrival of a deadlier evil bringing him down, and thus gave the Rangers more breathing room to recover.
Lord Zedd's personal Zord, Serpentera, is powerful enough to destroy an entire planet, can fly across the Galaxy almost as fast as the Rangers could teleport, and was more or less indestructible when compared to any weapon at the Ranger's disposal. Its only weakness was its absurd energy usage. The only thing standing between the Earth and certain destruction is not the Power Rangers, it's the time between Serpentera's next recharge.
The skeleton Rito Revolto could have gone this way if he wasn't made a comic relief character: his destruction of the Thunderzords (previously known to be indestructible) was a moment of high "Holy Shit!" Quotient.
Any time a series' Megazord gets destroyed is scarring, but the most heart-rending is the destruction of the Thunder Megazord and the White Tigerzord. The White Tigerzord's head falls off before the body, trailing wires and sparking, and collapses backwards. The Thunder Megazord stretches an arm out imploringly - the other falls off, then the entire Zord falls apart bit by bit. The fact that Kimberly is screaming and sobbing at the sight certainly doesn't help matters.
Fridge Horror sets in as you realize nearly all Zords carry a degree of sentience...
Also, though the Rangers can get into a dire situation even at Zord level, the Zords usually convey this feeling of invincibility - Even when fighting a losing battle, it's usually a "more hit sparks than usual" situation. Not to mention the fact that they are (a) quite sentient, and (b) the rebuilt forms of the original Dinozords. This is also before pretty much any season finale might have Zords blowing up. These almighty living machines who'd been as much a part of this world as the heroes themselves from day one, who'd survived two "oh god the Zords are being destroyed" moments ("Green With Evil," where they got better once contact with Zordon was restored, and "The Mutiny," where they were transformed into the Thunderzords) were just falling apart, section by section, with the Rangers, Zordon, and Alpha helpless to do anything.
Said scene actually damaged the Thunder Megazord costume. DairenOh doesn't appear in other Super Sentai crossovers outside of stock footage, with the Red Dragon Thunderzord suit used instead.
The episode where Kimberly and those two guys are working in a cab service and then the villains zap their cab so that it turned into this freaky mutant monster cab thing called Crabby Cabbie. Said girl and guys got stuck in its stomach.
A potential Nightmare scenario that was thankfully avoided: When rescuing Kimberly, Bulk, and Skull from Crabby Cabbie's stomach, Alpha 5 said he would have to do it very carefully, since it involved separating their molecules from the monster's. If the separation wasn't done correctly, he explained, they could end up with Kimberly or "Kimberbulk." Paging Dr. Brundle...
It didn't happen often, but every now and then Rita would really lose her temper and lash out directly at the Rangers with her magic. When she did, the Rangers went down hard every time; even the Ultrazord wasn't able to withstand her anger. And you never knew when she would snap and strike them down, but when she did, you finally understood why she was the one everyone was afraid of.
Master Vile creates the Blue Globbor in his debut three-parter by regurgitating a green egg, then cracking it open with his staff, resulting in this blue slime pouring out, causing the (originally mini-sized) Globbor to emerge before growing to normal size. It's pure Nausea Fuel.
