Animorphs gave us the nightmarish puppetmaster Yeerks, the Taxxons, insane-with-hunger giant centipedes (who are also cannibals and will eat themselves if they are injured/hungry enough—and they're always hungry), and the Hork-Bajir, benign but enslaved bladed lizard men (who started out as mooks but turned out to be really sympathetic), as well as morphing in general and the terror at getting stuck in animal form—or, worse, between forms—in particular.
In one book, we get to see how a Taxxon thinks—which is basically a never-ending, overwhelming sense of fear of starvation. Oh, and Tobias attempts to eat his friends while in this morph. Not fun.
In the same book, there's also the sheer claustrophobia and single-mindedness. It's like you can never get out. Way to drive home the hopelessness of the Taxxons' situation.
And then we realize that there are some Andalites who got stuck that way. Forever. Arbron, at least.
Remember that fight scene where Marco's bone breaks through his skin? Or the one where Jake struggles with smashing a Hork-Bajir's head against the wall while his tiger intestines fall out of his body? The only fight that isn't described in explicit detail is the one where Visser Three eats Elfangor alive. Because hey, this is a kids' series, after all.
The fights got more graphic after the first couple books. It's as though the author wanted to censor out the worst of it, then realized that the series was already horrifying to kids anyway.
To see Cassie, usually the most in-control of her morphs, so panicked and freaked out by the experience of the termite morph that she demorphs while still inside a piece of wood is chilling.
Another insect-related example: All the books have fight scenes that are gruesome on various levels, but there's one that takes the cake: The Forgotten, book eleven, has Rachel falling unconscious on top of an ant hill in the Amazon rain forest in bear morph. They then start eating her alive. Jake and the others are, of course, horrified when they see this. And when Rachel wakes up and demorphs, she screams the entire time.
And the fate of David, the sixth Animorph. He turns against the Animorphs, so the team decides to trap him in rat form and leave him on an island with dozens of other rats. People boating by months later could still hear him mind-screaming.
Not to mention that Rachel and Ax are forced to wait the two hours with him to make sure he's stuck. As Rachel said, you can't block out thought-speak, and she notes that she would hear his screams and pleads and threats everytime she fell asleep.
His environment and how he's forced to adapt to eating garbage in #48 The Return are disgusting.
David himself crosses into nightmare territory when he remorselessly murders a hawk he thinks is Tobias, and reveals he is fine with killing the rest of them to get them out of his way. The worst part? He almost does.
The fate of Saddler, Jake and Rachel's bratty cousin. It's never explicitly stated, but strongly implied that David put the already suffering boy out of his misery to take his place and have a family again.
The nightmare-within-a-nightmare ordeal endured by Rachel in the same book, though she eventually figures out it's all a hallucination caused by Crayak.
All the worse because there's some evidence that her transformation took place not just because she wanted it to, but because her friends needed it to, even if they wouldn't admit it (Jake, for instance, expresses guilt at using Rachel for the dirtiest work - given where it gets her, you can't blame him). Rachel herself doesn't really become conscious of this until her little tete-a-tete with Crayak, a conversation that culminates in an argument with Cassie. When Cassie starts (as usual) objecting to Rachel's latest morally troubling but wholly necessary choice:
Cassie': I don't think you can do it a second time.
Rachel: (snapping) You know what, Cassie? I don't think I can, either. So will you do it for me?
Cassie: (taken aback) I...I don't...
Rachel: I didn't think so.
Rachel imagining sticking a fork in David's ear was incredibly disturbing. We may need an Ear Scream page just for that scene
"I fought back a nauseating urge to twist the fork, to make him squeal in pain." Rachel, ladies and gentlemen. Making plastic utensils a source of horror in one easy step.
Imagine being one of those seventeen thousand Yeerks that Jake flushed into space, at first confused, then in a sudden stab of terror realizing what's happening and completely helpless to stop it...
Fortunately for them, they have next to no sensation, so by the time they were aware they got spaced, they'd probably be dead.
Megamorphs #3 where Jake gets no-nonsense shot in the head and dies, Cassie briefly snaps, and Rachel gets blown in half by a cannonball.
In The Andalite Chronicles, a Quantum Virus is mentioned. It can be programmed to target a species, and basically wipes them from existence at an atomic level. Later seen in the Hork-Bajir Chronicles. Just as terrifying as it sounds.
And most of the Andalites think the reports of its use are just Yeerk propaganda.
Made all the worst by the fact that Marco is supposed to be the cocky, funny guy of the group, and his only response to that experience is to just collapse into tears.
The beginning of the first book, when we first realized this wasn't Goosebumps. The kids meet a dying alien that gives them the power to morph. In a gentler story, Elfangor would have become the quirky alien mentor or something. But since this is Animorphs, guess what? Visser Three turns into the first of his many horrendous morphs and eats him.
Pieces of Elfangor fall out of the Visser's mouth and Taxxons, who were waiting at the Visser's feet, jump up and eat them.
Which later becomes even more horrifying once you realize that Tobias was watching his own father being eaten alive, and most likely remembers every single moment of it.
Don't forget Alloran. Not only is he himself being forced to eat his previous protege, but he's from a non-meat-eating species, making the experience a whole new level of horrifying. In fact, Andalites are most likely a prey species. (V3 may very well have gotten into the habit of eating his Andalite enemies just to bully Alloran.)
Visser Three. Sure, he's a bad guy and he's an egomaniac. But it gets worse. It gradually becomes clear that his Affably Evil personality is only a masquerade which does little to hide his psychopathic tendencies. He's a violent nutcase who flips out at little-to-no provocation and will often execute his own subordinates, or torture them for weeks, for no other reason than the fact that he's pissed off, which is pretty much all the time. And he's got a variety of morphs hand-picked for the fact that they can kill you in the most grotesque ways possible (one of them can shoot acid, for instance). For Christ's sake, the guy has a personal collection of torture instruments from all over the galaxy for entertainment.
Even worse is his disturbing fondness for eating his victims while morphed into giant predators.
In a book where Jake is infested with Tom's old Yeerk, we're treated to a memory of the real Tom, who's become a broken, empty husk of a person, mentally sobbing and begging the Yeerk in his head to leave Jake alone. It's not exactly fun to read.
In "The Escape" Marco encounters Visser One while pretending to be a human controller, who reveals that its host (Marco's mom) is currently having a mental breakdown upon viewing her son and believing he's been enslaved, but the Yeerk has simply learned to ignore her pleas.
Visser One: You must learn to control your host more completely. My own host is in here creating an awful racket, but I do not let her weeping and wailing disturb me.
Marco: No, Visser. I will try harder to control my host.
Rachel morphs into a starfish, gets split it half, and ends up as two Rachels. One is a complete psychopath. The other gets to tell the last chapter, when they get reunited, and is terrified that the bad one is a part of her again.
Crayak. He's an Eldritch Abomination portrayed mainly as a disembodied eye that lives in the space between dimensions thanks to that incident with the black hole. And his sole purpose in life is to destroy as much life as the Ellimist will let him, all for kicks. Applegate talked about him, saying she wanted him to represent true evil, on an existential level, much like Sauron from The Lord of the Rings.
When the Ellimist was talking about the origin of his battle with Crayak, he told the gang that Crayak was expelled from a far-distant galaxy hundreds of millions of years ago by an even more powerful entity. Also much like Sauron.
The scariest thing about Crayak? His introduction. Jake experiences the Yeerk dying in his brain, and, just like that, is pushed face to face with something so awful, so utterly evil, that it nearly breaks him... with no explanation. None. Ellimist hasn't even shown up yet, and Jake won't know what it is for a very long time. He mentions having awful nightmares about it in The Attack.
The Howlers are literally children, and believe their constant mindless killing of other races is actually a game, and they enjoy it.
And the fact that Jake doomed the entire species to nonexistence by downloading his memories into their hive-mind; the memory of Jake kissing Cassie manages to get through before Crayak erases the squad the Animorphs are fighting, and the Ellimist tells them that the next time Crayak turns the Howlers loose on another species, they will try to kiss the aliens instead of kill them. One imagines it wouldn't take Crayak long to get rid of them for good after that.
The One. Aximili, of all individuals, being reduced to just another tiny part of an unbelievable being's consciousness is already creepy as all get out, but then there's the mouth...
For a series about aliens, Animorphs made some of Mother Earth's own children into pure horror for many a child. In The Predator Marco describes Jake morphing into a lobster and his face "exploding" into valves. Imagine seeing a human being's face turn into this.◊ Or in The Suspicion, Cassie describes a giant (from her perspective) wolf spider◊.
After having read the series for a while, one begins to ponder why one ever thought animals to be sweet, cool, or indeed anything other than absolutely deadly. Which is appropriate, seeing as the characters (especially Cassie) have the same realization.
Apparently the Body Horror was so bad even the author had nightmares.
Marco exhibits some slightly Machiavellian tendencies, especially in the later books. He often talks about "the straight line". It's a personal philosophy of his; there is a straight line from A to B, the simplest way to do it, ultimate efficiency. Sometimes following this line forces you to do some things that you'd rather not (like kill your family), and thus, you diverge from the line. But the line is still the line, and for that ultimate efficiency, it must be followed through (to the hilt, so to speak). This philosophy led him to plan in detail - and execute to completion - a plan to throw his own mother off a cliff to kill the alien in her head...
Marco eventually realizes that he can't go through with killing his own mother but Jake does it for him, while holding down a screaming and crying Marco.
Tobias getting tortured as a means to fool the yeerks into thinking the demorphing ray doesn't work. Tobias lost a sizable chunk of his sanity there, and it's really obvious.
And everyone basically knew it was going to happen. Jake, at least, realised that it was the ultimate plan before Tobias himself did, but needed to wait for Tobias to volunteer. The team basically forced doubt and indecision aside and threw their friend in the grinder to protect themselves, showing a lot less consideration for his safety and well-being than they do for many of their enemies.
Forget the torture: the torturer herself, Taylor, aka Subvisser Fifty-one, is pure nightmare fuel. The result of an insane Yeerk infesting an already unbalanced girl, she see-saws back and forth between the two personalities, sometimes the cold, power-hungry subvisser, sometimes a broken Alpha Bitch, always sadistically insane and dependent on other people's pain to get through the day.
The Yeerks themselves. An alien slug forces its way through your ear canal and wraps itself around your brain - then takes over controlling your body while you are trapped helplessly in your own head.
The implications of the Yeerk going about your life for you. Have a husband or boyfriend? Do a lot of casual hooking up? Anything in between? Well, your Yeerk is going to go on with it. Which means you don't get to decide when you have sex anymore, or how, or who with.
Forget sex. Your body will be forced to kill people, or worse, force another slug into their head. You will betray everyone who ever loved and trusted you. And there's nothing you can do about it.
One of the worst things seen about the Yeerks really drives home how horrible it would be to be a controller. In the Yeerk pool, the humans whose Yeerks are feeding in the pool are put in cages. Most of them either cry, scream, curse, or rage at the guards, but some just sit there in silence, and wait to be called back, because they have literally lost all hope of ever being free again.
Rachel transforms into a shrew, and is very nearly overwhelmed by the instincts. Particularly notable is the bit where she's being held by the rest of team, and doesn't actually react to them.
The fate of the Venber in The Extreme. The Yeerks had cloned an extinct alien species, the Venber, to make shock troops specially adapted for cold temperatures. The problem is a) they can't survive in temperatures above freezing, and b) the Venber clones are being controlled to follow programing instead of a Yeerk. So in the book's climax the kids run into area where there a large amounts of powerful lights. And the Venber follow. They melt. And even as they slowly die their programming forces them to twitch and writhe around and try to follow the Animorphs as their body liquefy. Yikes.
From the same book, Ax takes down a Taxxon while the others are in fly morph. They can't see well, but they can sure as hell smell something...
The time that Marco got swallowed by a bird whilst in wolf spider morph. Also, how about in The Message where he gets bitten in half by a shark whilst in dolphin morph.
The school's principal: Stock children's-fiction villain, and host to the mind-controlling parasite aliens' leader, being given back 'the helm' inside his own head, and allowed to beg for his family's life before the Visser Three. He falls over and slurs and drools because he hadn't had willed control of his body for longer than it takes the Yeerk to feed in years.
The animorphs' ages are never specified until near the end of the series. Those violent battles, having their limbs torn off, being forced to kill innocents, watching the people around them being puppetted by alien invaders? They started when they were thirteen.
Basically, the only militia to defend the world against an Alien Invasion is composed of six kids who would not be able to join the actual Army. Nice.
The description of the man's gangrenous leg that Cassie had to cut off in #44, The Unexpected.
#39 The Hidden, when Cassie sees an ant morphing into her. This is horrifying enough, but we then realise that the ant!Cassie is screaming because it's lost its Hive Mind and become an individual. Also, the Buffa-human is just creepy.
In The Experiment, the Animorphs have to infiltrate a slaughterhouse using cow morphs. Ax, before he has a chance to demorph, is herded onto the killing floor, and sees the cows ahead of him in line die, one by one, as the butcher approaches...Even after he's rescued and demorphs, Ax "could not stop trembling...could not stop shaking."
There's the Bad Future seen in #7 The Stranger. Apparently the Animorph-Controllers ate Tobias with barbeque sauce. Yeah.
In #31, Jake orders Ax to torture Chapman. Ax still is still an adolescent like the other Animorphs. To make it worse, throughout the night he can hear Melissa Chapman crying for her father. It is such a terrible experience that Ax tells Jake will will never do anything like it again, saying <I will gladly fight this Controller and even, in fair battle, kill him, but I am not a torturer.>
Ax is so furious at Jake for making him do this, that for the first time in the entire series, Ax coldly refers to him as just "Jake", no prince title.
Hell, the covers themselves are scary!
The Mercora: six kids stuck in the past decide to allow an entire sentient species to die, because if they don't humanity will never arise on Earth.
In The Andalite Chronicles, during the Time Matrix sequence, Elfangor and Loren walk into a Mickey D's and see someone who's face is practically made of acne!
In "The Underground" it's revealed that Instant Maple & Ginger Oatmeal drives Yeerks insane. The Animorphs' first encounter is when a host throws himself out a window trying to commit suicide. Later on the Animorphs shoot open a barrel of oatmeal in a Yeerk pool, and it's implied the Visser will simply let the thousands of Yeerks affected die.
Rachel taking command in "The Weakness," where she decides to fly a jet into a tower controlled by the Yeerks. Thousands of Yeerks die in the inferno, and before that one of her plans leads to Cassie being captured and tortured by the Yeerks.