When Twilight's book is telling how Princess Luna transformed into Nightmare Moon, the illustration suddenly goes from Luna's eyes closed in sadness to Nightmare Moon's eyes open in utter hatred.
The Ursa Major that was seen at the end of Boast Busters. The way it's a grotesque mix of at least two creatures that are terrifying on their own is petrifying.
The Ursa Minor as well. It's big enough to destroy Ponyville in a matter of minutes, lives in a cave in the Everfree Forest right next to the town, and all it takes to wake it up is two young, curious ponies stumbling into its cave. Trixie's reaction to hearing it was only a baby is very justified.
How about the first time when Fluttershy was so relieved to have found her friend, she starts talking to her, and then BAM. Realizes she's stone. That's just about the scariest thing. Even scarier, if you recall Discord after being freed from stone, he could see and hear everything from around him. It must be the saddest thing in the world to have a friend worry about you in which all you can do is watch. This should be labeled in And I Must Scream
Try listening to the background music with headphones on as the Crusaders enter the forest.
The way the Cockatrice itself rises up out of the bushes is quite unnerving.
Though on the bright side Zecora who lives in the forest 24/7 probably has potions to cure being turned to stone on hoof at all times just in the off chance she runs into a Cockatrice. So if Fluttershy had failed, Zecora could possibly have been able to reverse it.
The Show Stoppers
The Cutie Mark Crusaders getting caught on Pinkie Pie's candy machine in "The Show Stoppers" is really frightening, especially with their bodies disappearing in the rainbow goop and only the Eye Scream left visible. They came out intact (albeit sticky) but still.
Becomes more horrifying if you happen to have listened to (or read the fic version of) "Rainbow Factory".
A Dog and Pony Show
The first time the Diamond Dogs show up, hiding in a bush and talking in a Gollum-esque voice and you have no idea what they look like.
The Cutie Mark Chronicles
A lot of fans were creeped out by young Twilight losing control of her powers in "Cutie Mark Chronicles". She is writhing and twitching inside the bubble before Princess Celestia comes and calms her down. She was in pain. Let's just be glad Celestia came when she did.
Party of One
Pinkie Pie temporarily going nuts in "Party of One" was legitimately spooky, bordering on pure terror. The odd expressions and the violent spasms make it worse. The creepiest part is, after they say that it's her birthday, she is just to normal, like it never happened.
Pinkie was starting to hear voices in her head, and believing they were coming from inanimate objects, signs of schizophrenia (and Funny Schizophrenia is averted as much as possible on a kids show). After one day of being alone. Still not bad enough? The voices in her head were telling her that her friends were losers and she shouldn't speak to them ever again.
It's bad enough when you see the build-up. Put yourself in Rainbow Dash's shoes for a second. This is one of your best friends, and you go to get her for a surprise party. You expect her to be her usual, cheery self, and instead she's... Gone crazy. Not the fun kind of crazy she usually is, but scary crazy. You don't know what happened, but your best friend is suddenly not herself, in a very disturbing way. And she's replacing you with a bucket of turnips that she thinks is talking to her. How do you react to that?
Fans have left comments on Youtube saying they believe this episode inspired the infamous Cupcakes. (In actuality, this episode was written beforeCupcakes was written, but aired after said fanfic was released.)
While Rainbow Dash is fleeing from Pinkie Pie, she hides inside a bell at one point. Then you see two eyes... on the clapper. It's Pinkie Pie as the clapper.
"The Return of Harmony", part 1: When Discord first begins to awaken, we see his heart beating inside of his stone-cold prison, with a black aura pulsating in and out of visibility each time the heart beats.
Discord gets up to some pretty disturbing antics. Robbing unicorns of their horns and pegasi of their wings, brainwashing Twilight's friends into becoming the opposite of their true nature, tricking Rainbow Dash into abandoning the game with a vision of Cloudsdale being destroyed...
Although it's not dwelled upon, the taking of the wings and horns has some Body Horror implications. Imagine having a couple of your limbs vanish.
Rainbow Dash's encounter with Discord gets even creepier when he produces a box that he claims contains her wings. Granted, we don't see Dash open it, but the idea that her unattached wings could be just sitting there inside a sparkly, neatly wrapped gift box adds new levels of unsettling to Discord's antics.
Pinkie Pie's Mind Rape is probably the creepiest of all, with demonic balloons laughing at her and circling her while she begs them to stop.
Even worse when Discord removes his head so his talking face is on the balloon. The faces of Pinkie's friends showing up on the balloons too is unsettling at the very least.
The others suffering from Mind Rape is rather unnerving as well, not only did Discord reverse their personalities but metaphorically raping them as well.
The animals with the long spidery legs at the beginning. They just look... wrong.
The worst part of it all is Discord's Faux Affably Evil personality. It's just so... disturbing. He loves what he's doing to them. He loves performing Mind Rape on the mane cast and he didn't even need to do the mundane version; as he showed with Fluttershy, he could've skipped straight to the instant supernatural kind. He did it because he wanted to. No wonder Celestia and Luna imprisoned him instone for eternity!
Not only that, this shows that he's not even trying, and yet he still won! Who knows what he's capable of when he starts getting serious?
And to think that he ruled Equestria before Celestia and Luna came along and turned him to stone with the Elements of Harmony.
Another bit worth noting is part of the stained-glass window with the puppetmaster Discord. Him controlling the strings of the obviously-in-pain ponies is creepy enough, but then you notice he is dangling the ponies above a pit of fire. Do we even want to know what happened there?
Those who study Tarot would definitely see the resemblance between this image and the Devil card from most major decks; the ponies being "chained" by the puppet strings, Discord's somewhat goat-like face, even the pit of fire bit can be found in the standard variants of this arcanum.
The Return of Harmony: Part 2
The way Discord is defeated. He writhes in pain as he is turned to stone. Afterwards, the statue falls to the ground, the camera showing a close up of his horrified face.
Discord's World of Chaos was getting progressively worse by the minute, and he only ruled Equestria for a few days max. With what was implied the previous episode (Ponies being held by puppet strings over a pit of fire along with pained, horrified screaming), it's a very good thing they stopped him when they did.
Discord: And these are only my first changes!
The "slender bunnies" are freaky enough, but if you look closely when they stampede across screen before the final confrontation with Discord, it looks as if they don't have eyes.
What made Lesson Zero so effective was how much it played with our expectations after The Return of Harmony. For instance, who would've expected any of this?
On the inverse, some people found it scarier because it came after "The Return Of Harmony" and it made it seem like Twilight was psychologically damaged by Discord.
For starters, quite a few people found Twilight's mental breakdown to be more than a little unsettling. Also, she seemed to have the power to brainwash ponies. She literally managed to cause all of Ponyville to descend into chaos just with one spell.
The spell also seems to make ponies eyes roll back into their skull rather slowly.
Fluttershy apparently breaking a bear's neck. It turned out she was only doing something chiropractic on it, but it looks horrific at first.
Spike's claw cramp. It looks like he has a grotesquely broken wrist, and the sound effect doesn't help.
The short moment where Twilight notices a few fillies playing jumprope, then she imagines them laughing at her, in an almost literally hellish background (see the page image). e episode.
Zecora telling the fillies about Nightmare Moon. She blows some green dust and illusions of Nightmare Moon appear, looking very vicious.
Princess Luna's actual entrance. She flies in with two grim looking ponies, with lightning flashing in the background, making her look pretty intimidating.
This is part of the conflict for the episode—as it turns out, one thousand years ago, the standard royal etiquette was to be intimidating.
After Pinkie Pie's third misunderstanding with Luna, the princess brings all the fake spiders to life! Which immediately overwhelm one of the ponies, knock her down, and crawl all over her. To anyone who has arachnophobia (or just doesn't like spiders), this is the creepiest part of the episode.
Luna finally gets furious after spending the whole episode being treated like a monster, complete with Glowing Eyes of Doom and summoning an ominous storm out of nowhere! A good lesson on why ticking off a Physical Goddess is a very bad idea!
Apple Bloom appears to have been on the verge of dying of exhaustion.
A ponified Jesús Quintana, in the same building as two groups of foals!
May The Best Pet Win!
While the Mix-and-Match Critters of the pets in Rainbow Dash's dream aren't that scary and are actually kind of cute, having Opal coming out of its mouth is truly frightening.
It does not help matters that Opal in that scene reminds some of Charlotte.
The Quarry Eels. They aren't particularly monstrous, but damn they're fast, and not one of Dash's prospects manages to avoid being chomped. This doesn't stick (the bat pulverizes one from the inside out), but they lair in otherwise normal holes in the wall, and they give no hint to their presence.
Sweet and Elite
This episode features a split-second closeup of a Salvador Dali-esque painting of ponies being horrifically stretched and melted. And Rarity approves of it!
The Secret of My Excess
Spike's display of his nictitating membrane after Twilight leaves him be can be creepy to those unfamiliar with reptilian biological mechanisms. We've come to accept these characters as humans rather than animals like they really are. This scene is similar to watching someone you know as being as human as you are slowly turn into something not human.
While grown up dragons in the show have a generally cool appearance, Spike's grown up form looks more menacing than the ones in "Dragonshy" and "Owls Well that Ends Well".
Hearth's Warming Eve
The Windigos. Ghostly horses that feed off of negative feelings and distrust. The blizzard they caused further fueled the emotions that fed them. Close to the end of the episode, their cold became so intense that it froze the leaders of the three tribes alive, right where they stood. And apparently, they're still around, meaning that if you ever let your negative feelings get the better of you, they'll be waiting.
The scariest part is when we first see them, there's nothing explained about them. Until the end, we have no idea what they are or what they're up to.
The moment they first appear... the camera zooms out past the cloud layer and we see these huge horse-like creatures with flashing eyes neighing loudly and clearly causing the giant, kingdom-destroying ice storm.
It gets better with research: they're either named after an Algonquin legend of evil spirits that could enter people and turn them into cannibals, or after Ithaqua the Wind-Walker, also known as the Wendigo: an Eldritch Abomination also known for causing horrible, deadly blizzards, and a character from the Cthulhu Mythos. Now that is a Shout Out the original primary demographic is unlikely to get.
Family Appreciation Day
The Timberwolves, a very eerie design of wolves made of wood. Actively malevolent, and practically immortal (regenerative abilities) to boot, it's another rare time on the show where Talking the Monster to Death won't work.
The "signs" that Granny Smith is on the lookout for, when they haven't been explained, can be a little unsettling. The Timberwolves' howling, the ominous weather, the crows and ravens forming symbols in the sky... it feels like Black Magic rather than an apple harvest.
Even after they're explained, there's still something terrifying about a ominous storm coming out in the middle of an average, sunny day while crows squawk and winds that come from nowhere blow the leaves aside to show the apples changing color.
The scene where Pinkie Pie is searching frantically for the twins after putting them to bed looks like a scene out of a horror movie. She finds Pumpkin Cake (the filly) in a closet, chewing on a rubber chicken in increasingly-more-jarring slow motion. She goes out into the hallway to find Pound Cake (the colt). It's dark, the curtains are blowing, and there's weird music playing and she keeps hearing Pound Cake's laugh everywhere. It brings to mind some of the horror stories about babysitters alone in the dark...
That scene was filled with references to horror movies, so it's only natural it would be scary.
The blowing curtains also imply that there is an open window somewhere, and Pound Cake has just started flying. And the Everfree Forest is never far away
Well, considering that scene was directly lifted from the "dead baby crawling on the ceiling" scene in Trainspotting, it's naturally horrifying.
There's also a massive amount of Adult Fear in that scene. Taking care of a child and then losing them is one of the worst feelings ever.
The entire Daring Do story is quite shockingly violent for this show. The worst is definitely when she's tied to a slab in a room filling with sand and spiked walls closing in, while spiders and cobras crawl over her.
Ahuizotl's introduction. It's not immediately apparent how off his anatomy is until he starts talking, and when he does, it's in Pinkie Pie's voice, which is immensely jarring.
which says nothing about his real life origin: 
Hearts and Hooves Day
Apple Bloom's Imagine Spot about Ponyville sliding into chaos without Cheerilee teaching and Big Macintosh helping at Sweet Apple Acres is amusing, but it also features a disturbing shot of a severely-emaciated Lily.
A Friend in Deed
It was somewhat unsettling to see Pinkie Pie squeeze one of her eyes through the keyhole of a lock.
Because Nightmare Moon, Discord and the Windigos aren't bad enough, Cerberus apparently guards a gate that holds all manner of evil creatures, and there's nothing stopping him from running off whenever he feels like it. Not only that, but according to Twilight the place is Tartarus - which was more or less Hell in Greek myth, as the lowest region of the underworld where the souls who received punishment were sent. Given that Cerberus guarded the underworld in Greek myth, and the MLP production team has snuck quite a few mythological references in, the fact that we're more or less introduced to Hell is unsettling in itself.
The ending of the first half. Specifically,thelast20-oddseconds. Seriously, the only thing that kept fans from freaking out completely is the fact that the second half of the plot started almost immediately.
When Chrysalis unravels her Cadance disguise, there's a short bit where, for a second, she has an Eyeless Face, and then her eyes suddenly open with stretched out sclera. This is then followed by an Evil Laugh with an extreme close-up of her face◊. Yikes.
The shot directly after that, when her shadow falls over Cadence, looks absolutely terrifying!
Celestia finding herself trapped in a chrysalis.
The moment where Twilight was trapped in the abandoned mines of Canterlot, and Chrysalis!Candance showing herself on the crystal walls, taunting her and telling that Nopony would ever find her... Also her face when she reveals herself to Twilight can count!
Many of Chrysalis's expressions while disguised as Cadance, notably her crazed looks during "This Day Aria".
The entire episode save the ending, pretty much. Just for starters: the Changeling Queen's true appearance.
For those with trypophobia (the fear of clustered holes and/or holes in bodies), her limbs and horns can be downright horrifying, on top of her already-unnerving appearance.
Not to mention those with entomophobia and how the changelings looked like a hybrid of a pony and a housefly.
Queen Chrysalis revealing her true form. Especially the spot directly afterwards where her shadow falls over the real Cadance. The shadow seems specifically intended to make her look as terrifying as possible!
Queen Chrysalis leaving Twilight trapped under Canterlot castle. The entire scene is absolutely terrifying!
Especially before we know where Twilight is and what's going on. The darkness and reflections are terrifying!
The changelings cornering Dr. Whooves, Carrot Top and a baby foal in an alleyway.
The Crystal Empire
Just the reaction of the Crystal Pony Twilight talks to when thinking about King Sombra. The way she says she doesn't want to remember King Sombra's reign involved outright slavery and was so horrible that Celestia and Luna intervened, turning him to living shadow and sealing him in ice for eternity. Keep in mind, unlike Discord, this guy is a unicorn, not some Reality Warper. Just what kind of monster is this guy that the Princesses were more thorough disposing of King Sombra than they were of Discord?
Sombra licks his lips when he sends a dark crystal at Spike in an attempt to intercept the Crystal Heart, possibly to spear him or otherwise eat him.
When you think about it, Sombra came this close to actually killing a major character. And the fact it was Spike makes it even more disturbing. No other villain has accomplished that.
King Sombra is actually killed by being shattered into pieces. He doesn't get turned to stone, he doesn't get blown away, he gets literally blown to smithereens. This may be one of the few times a villain actually dies in the show and one of the most disturbing deaths ever.
Worse? If you look closely, some parts of him hadn't turned to crystal when he shattered, including his horn which is thrown at the camera.
King Sombra's lack of a backstory adds more fuel to the fire. Who is he? Why is he so obsessed with the crystal ponies? How did he become so powerful that it took two God Emperor level princesses to seal him away, and even then it wasn't enough to stop him?
King Sombra, period. The guy doesn't have much screen time, but he manages to make every second of it absolutely terrifying.
More terrifying than anything else is how Genre Savvy he is. Dangerously so. The Crystal Heart is the one thing, asides from direct intervention from two god-like beings, that could stop him. What does he do to prevent others from getting it? Asides from making everyone who knew about it forget it existed, and removing any record of it's importance? He put defenses around it. What kind? First, he puts it in a secret passage that a spell only he knew could open. Had Twilight not seen that spell from Celestia, it would be impossible to reveal. Second, he puts a door in the way that, if the same spell is used to open it, traps the intruder in a nightmare that is inescapable unless someone else wakes them up. And the door itself is a total jerk and will toy with anyone who tries to enter it. Last but not least, at the top of the tower where he hid the Heart, there's a spell that, when triggered, traps the intruder in a barrier that is extremely difficult to escape from. Said barrier will pull the victim back in if a teleport spell is used. More importantly, it tells Sombra that someone has reached the heart. Unlike some villains, he took every reasonable step to ensure that no one could get the one thing that could stop him, since it couldn't be destroyed (said object having the power to aid him in conquering the world probably has something to do with that). Not bad for a villain with only a couple of minutes of screen time and very little dialog.
And the stairs. Hundreds and hundreds of stairs. After climbing down an absurd number of stairs, anyone after the heart would then have to climb up even more stairs to reach the heart. So, anyone who tries to claim the heart will be exhausted by the time they reach it, and unless you recklessly jump off the top of the tower like Spike did, the only way out is going down, then up, those stairs again. And by the time you get there, Sombra would already be waiting for you at the entrance, because his spell would tell him you've taken it. Seriously, King Sombra takes Crazy-Prepared and Dangerously Genre Savvy to new and frightening heights.
Let's put this into perspective. The only reason he was defeated was because Spike didn't listen to Twilight, and went after her.
Twilight's worst fear. As Twilight opens the door and it reveals Canterlot, rather than a part of the Crystal Empire, the way Celestia treats and looks like towards Twilight surely would creep everypony out!
The very idea of a door that leads to your greatest fears, brainwashing you into living out your worst nightmare. Mind-Control Eyes and everything.
Also, the wall had one of those mosaic's that depicted Equestrian history. What did it depict? Sombra winning! Not only is it pretty frightening in just the idea, but the imagery is truly frightening, more than even Discord's!
Looking closely, it had Twilight being sent hurling down onto crystal spikes that are engulfed by a pit of flames while a truly terrifying depiction of Sombra looks on. So yes, he most certainly managed to outdo Discord in the 'horrifying things depicted in stained glass' category.
Spike's worst nightmare. We don't see, but we can hear it, and if it's anything like Twilight's, it was horrifying. What was it? Twilight dismissing him, saying she no longer needed him. Less than ten seconds, and he was brought to tears by it.
Heck, the mere thought that a psycho like Sombra even had this spell in the first place!
As the episode progressed, the field began to flicker back and forth as Cadance loses strength, flickering between serene blue to hellish orange, reminding the audience that Sombra is just waiting for it to fail so he can retake his kingdom.
Then at the end, when it does fail, the Crystal Ponies are instantly terrified at the realization King Sombra's returned. One of them has a Freak Out from the simple realization Sombra's back and runs for her life... right into King Sombra himself who gives her a Slasher Smile. The tone in which she says his name is one of complete and utter fear. This scene isn't scary because Sombra's in it (though that is scary), it's the sheer terror he's able to inspire in his slaves.
Unrelated to Sombra, Pinkie Pie is revealed to own skintight suits of her friends. Fluttershy, for one, is visibly disturbed by this development, especially because Pinkie was wearing a costume of her.
Too Many Pinkie Pies
Similarly to "Lesson Zero", the thing that made this episode so effective was how much it blew our expectations away. It was being advertised as a very silly and lighthearted episode. And it was! Until the climax. The fact that it came after "The Crystal Empire" didn't really help at all.
Twilight dispatches the Pinkie clones by zapping them with magic that inflates them into bizarre fleshy balloons that then pop into pink sparkles. It's very sudden and freaky, and the real Pinkie was in danger of being transformed by accident.
The slightest show of distraction from a featureless wall... and BOOM! Headshot. One right after another... until only one remains. And as mentioned below a couple pinkies distract the others in ways that you would be very hard pressed to ignore.
Another clone blows into one hoof, and the other sprouts small, alien-like fingers. It'd be enough to freak Lyra out, if not thrill her to some degree.
If you happen to be an introvert, then this episode in a nutshell. If you thought one impulsive, personal-space violating, won't-leave-you-alone person was bad enough, now imagine a self-replicating army of them, none of whom have the ability to feel bad about it.
Twilight misfiring her transmutation spell and turning a bird and a frog partly into oranges. The former was especially disquieting because when it gets hit we don't see the final product until the next scene, just a shower of feathers, making it look like it just straight up exploded.
Put yourself in Pinkie's shoes for a second. There's a swarm of hedonistic maniacs on the loose... and they all look and sound exactly like you.
Pinkie Pie almost tilting her head upside down to the ticking of the clock, while wearing a totally blank expression on her face. It's creepier than it sounds...
If somepony's duplicated by the lake, they can be sent into it. Odds are, Celestia's sealed something nasty in there.
Don't forget: all of the clones were shown to be at least minimally sentient.
Similar to in Magic Duel, we have something magical that can cause major problems if somepony cares to use it, with nothing keeping anypony from using it other than a bunch of thorns. And somepony used it before.
Seeing Trixie magically fuse Snips and Snails' horns together. The scene itself may be Played for Laughs, but there's just something really disturbing about it.
Not only that but there's also Spike being turned into a moving ball, bouncing unwillingly anywhere, and not being able to stop it. Imagine moving somewhere that's actually dangerous, and not being able to stop it. Think about that for a while.
It may have been an illusion spell making Pinkie Pie's muzzle invisible, coupled with one that held her mouth closed. Still nightmare fuel.
Pay attention at the very end. Pinkie with her mouth returned, breathes in. Which likely means that she was holding her breath that entire time, think that over... that is torment on a diabolical level. It's terrifying not just the implication of suffocation but the fact Trixie knew exactly what she was doing.
Some of her facial expressions give off the same vibe. Overall, this does a good job of showing Trixie is Not Herself and been corrupted by the Alicorn Amulet.
Even before the Amulet, there's her unholy grin right before the opening theme. You don't see her eyes or even her face, just her smile growing bigger and bigger...
The Alicorn Amulet in general. It has the appearance of a dark, glaring alicorn, and it corrupts Trixie to the point of wanting to take over all of Equestria.
One has to wonder: Who is that amulet representing? Does another much eviler Alicorn exist?
All of the above? The Alicorn Amulet made her go Drunk on the Dark Side. The idea something like that exists in this world is rather... terrifying to say the least.
And it's sold in a shady shop similar to Borgin and Burkes for anyone with money to buy...
The Cold Open, period. Even ignoring Trixie's creepy silhouette-Slasher Smile at the end, the shop is full of ominous items, including multiple bottles of green liquid with a skull-and-crossbones on the front. Poison canonically exists!
What's the first thing Trixie does upon exiling Twilight? Why, take over Ponyville and enslave its people, of course! She cuts off Ponyville's connections with the rest of the world, holds the Mayor hostage, and forces the ponies to dance for her and serve her every whim. And then she vows to overthrow Celestia and Luna themselves and take Equestria for her own. Good thing Twilight managed to get that Amulet from her before she was too far gone...
More terrifying is she very well could have succeeded. It's already been proven once that Celestia can be defeated by someone with sufficient power, at the Canterlot Wedding.
Even more terrifying is that this episode confirms that any pony with the will to do so can become that powerful. Scroll back up and reread the details about King Sombra. That is what absolute power can do to a common pony. Any common pony.
Most horrifying of all? It was originally believed, before the airing of the episode, that the Amulet was a piece of Sombra, something that he himself had made. Now it has been revealed that it's just a powerful magic charm. A charm that can amplify a moderately skilled unicorn like Trixie to a level where they could compete with Celestia or Luna in terms of power. Imagine if that amulet fell into the possession of Sombra, someone could stand up to Princess Celestia and Luna in a fight without it...
Fell into his possession? What if King Sombra was actually the end result of the amulet's influence? Good thing Twilight managed to save Trixie from it, otherwise Ponyville could have had the same fate as the Crystal Empire...
You can even suspect that the Amulet wasn't made in Equestria, or even as old as the archaeologist said it was.
The age spell is rather horrifying. Twilight's demonstration may not have been real, but Trixie's certainly was.
The fishbowl had a cloud forming inside it. Not through it, inside it. That means it was airproof. A bit longer, and suffocation could have been a problem.
The Cold Open, period. Even without Trixie's creepy silhouette-Slasher Smile at the end, the shop is full of ominous items, including multiple bottles of green liquid with skulls-and-crossbones on the front. Deadly poison canonically exists!
Sleepless in Ponyville
The show had the guts to show the decapitated body of the Headless Horse in Scootaloo's nightmare.
And the nightmare about the Olden Pony.
"Who's got my rusty horseshoe?"
Scootaloo's bloodshot eyes after the first night. That is some painfully detailed artwork.
Luna's glowing white eyes, coupled with the split second shot of her moving through the trees.
Luna has the ability to enter your dreams. Stop and think for a moment. This means nopony really has privacy, as Luna's always watching over all of her subjects. Run to it's logical extreme, it means Luna could even find and root out enemies of the state, or have you arrested for thought crimes. It's a truly terrifying power to have.
Here's a more terrifying thought: Equestria is a world filled with magical threats, and creatures of magic can attack from any avenue, even through your dreams. Imagine what kind of monsters might be out there, prowling your dreams, that are so dangerous that a Physical God has to patrol your dreams every single night to ensure that they don't come after you. Now, imagine what it must have been like while Luna was sealed inside the moon for 1000 years.
Keep in mind, Luna is a benevolent Princess who genuinely seeks to protect her subjects from their nightmares. As scary as she is, it'd still be making her a Ron the Death Eater to see her actively abuse this power. NightmareMoon? Not so much. And remember that one of the things that drove her over the edge was ponies sleeping through her night...
The destruction of the Twinkling Balloon and seeing the Mane Cast sans Rainbow Dash screaming in terror as they plummet to their deaths.
Made worse by the fact that Rainbow Dash herself was partly responsible for making that tornado that destroyed said balloon. She may be ultra competitive, but she would never be able to live with herself if said competitiveness ended up hospitalizing or even killing her best friends. If the worst had happened, even more then blaming Lightning Dust for her indifference to reckless endangerment, she'd have blamed herself infinitely more.
And this is Rarity's third time falling to her doom. She probably was having a terrible flashbacks the whole time she was falling, which would make this all the worse for her.
More Adult Fear than anything, but Lightning Dust's superficial charm (she gets along with Rainbow early on), callous disregard of others' safety (ploughing past her teammates and dismissing their plight as them "needing the practice"), astonishing selfishness (getting impatient despite one of the challenges not being a race and only caring about the twister when it costs her the leadership rights), and smug lack of empathy for Rainbow's understandable anger after the twister incident seem vaguely suggestive of psychopathy.
Applejack's entire family on a cart about to crash into the barn, after which the barn collapses on top of it and possibly kills off her entire family? Voilà, a family reunion to remember.
Spike At Your Service
Timberwolves are shown to be even more frightening in full CGI form. They can reassemble themselves even after being blown apart and can even assemble themselves into one gigantic being.
If you're reasonably genre savvy, you can almost see the regeneration coming because it's a kids' show and thus death on camera wouldn't normally be depicted this casually. However, now imagine you're a child. If the timber wolves really decide to come after you, even smashing them to bits won't stop them...
There is a constant sense of Paranoia Fuel surrounding Discord at all times. We know this guy could snap at any moment. Heck, even when he reforms Princess Celestia STILL warns the girls to keep the Elements with them at all times!
The fact Discord is immune to Fluttershy's stare is a little bit unnerving.
Discord states that he could hear every word that Celestia said while he was in stone. So does that mean that he was sentient for the past year he was frozen, still thinking while being unable to move? What about the thousand years he was first frozen?
The ponies (except Fluttershy) were willing to use what is essentially Mind Rape (in the form of a reformation spell) on Discord to keep him under control. Say what you will about Discord's villainy in the past, the very idea that the heroes of the show were ok with reducing a sentient being into a mindless slave is utterly horrifying.
Even if all the spell did was cause Discord to develop a conscience, that could be pretty horrible: Imagine that you have lived a life where you've done whatever you wanted without any concerns for others, and were then suddenly made aware of how much harm you have done to others, and were suddenly force to care. This sort of thing could be sanity breaking for an ordinary human with a normal lifespan. Imagine how bad you would feel if you'd been behaving that way for eons.
Due to having Fluttershy's cutie mark and destiny, Rainbow Dash is forced to care for various animals. At first, we see some funny scenes of her not being able to control the animals. Later on, the animals tie her up and dance around with silverware, threatening to eat her.
When Twilight finishes the spell, the Elemental Harmony Regalia suddenly blast energy at Twilight, causing the others to panick and scream. There is a huge flash... and then Twilight is gone, leaving a scorch mark. Additionally creepy, in that the scorch mark is in the shape of Twilight's cutie mark.
Not to mention that before she's teleported, she reacts with shock as to what's going on, then just closes her eyes and accepts it.
It is rather disturbing that Twilight can accidentally switch her friends' destinies such that they must live lives that are unrewarding and unhappy. Twilight definitely thinks so. Made worse that the others apparently don't remember their former lives, and are resigned to being terrible at what they think they must do.
Made even more disturbing since it likely means the five don't remember their families. Whether or not their relatives suffer the same memory alterations is unknown.
If Star Swirl the Bearded never finished this spell and he probably casted this spell a few times during his life. If so, can you imagine how many ponies must have been forced to live the rest of their lives doing something they're not meant to do? Not too mention, what if these occurrences had a hand in shaping key events in Equestria's long history?
Princess Celestia and Luna have been around for thousands of years, implying that Alicorns are immortal. Twilight's going to outlive her friends.
The sisters are the only princesses that have ethereal manes, so Twilight and Cadance might not have the same lifespan as them. Which now implies that the royal sisters will not only outlive their fellow princesses, but have outlived many of their own friends in the past.
Except, Luna is shown with non-ethereal mane in second episode. Twilight's mane might simply not kicked in yet.
Pinkie's line "It's no fun being me" from What My Cutie Mark Is Telling Me is absolutely horrifying. This is Pinkie Pie, fun ponified. We've seen how fragile her sanity is before, now think what would've happened if Twilight hadn't fixed things so fast. Twilight accidentally Discorded her friends.
Oh, and on that note, if the affected ponies don't remember the way things should be and everypony else is retroactively affected by the swap in friendships/family ties, what is Discord up to?
On the other hand, imagine what it must've been like for the Ponies' friends and family while the cutie marks were switched. Imagine what it must've been like for the Apple Family to see Applejack's life suddenly change. Or, how about Sweetie Belle? Or, heck, even Discord since Fluttershy is the only friend he's ever had.