The Titans are so scary, they almost looked like they were coming out of the screen.
The Titans. They were heartless killers, and it was implied in the opening song that the remains of their victims WERE LITERALLY EVERYWHERE. Considering they're portrayed as little more than mindless, hate-filled forces of nature that could not be reasoned with at ALL, only locked away, you can imagine that Hades would probably have had trouble keeping them under control if he had won. The only reason he got them to obey him was that he had a target for them, once Zeus was gone, who's to say that Hades could control what he had unleashed?
The Hydra: it's monstrous, no pun intended, and what makes it scarier is that it's like the one monster that gave Hercules trouble — even more than the titans, it was the only thing that came close to killing him.
It ateHercules alive! Followed by a horrifying belch. It's a good thing Herc happened to have his sword on him at the time.
It also stops to laugh at Hercules, and takes its time trying to eat him again when it has him pinned to the cliff, so it's also a kinda sadistic monster.
It's telling that Phil, trainer of heroes who has stared down countless monsters and horrors, immediately wants to turn tail and run when the Hydra steps out of the darkness.
Hercules' desperate struggle against the Hydra after it first revives. All we see are quick edits as Herc cuts and cuts, with more and more heads appearing. Finally, a look of pure horror on Herc's face as he stares down a massive FOREST of screaming Hydra heads.
Parents who watched Pink Floyd The Wall (who'd let a kid see it?) might find the Hydra's design unpleasantly reminiscent of this moment◊ from "The Trial", since both films have the same animation character designer (Gerald Scarfe).
Meg is seen dying in agony and then shown clearly as dead as Disney can get away with on a G rating.
Hercules' rescue of her soul showing his rapid aging gives both kids and adults nightmares.
As if these two aforementioned examples weren't enough, the Fates attempting to cut both threads of life with scissors is enough to make viewers young and old cringe and hold their breaths in fright. The first time, they are successful in cutting Meg's thread; the second time, Herc's thread glows and becomes invulnerable with his ascension to demi-godhood at the absolute last second.
For that matter, Hades getting pulled into the dead pool.
Hades' mood swings.
Hades, when he's angry, goes from controlled blue flames to out of control red flames. Hope you aren't terrified of fire.
His angry face is no picnic either. Just imagine watching the close-up on the big screen as a kid.
Hades: I can't believe this guy! I throw everything I've got at him, and — (Is cut off by the sound of rubber sandals squeaking, as he suddenly glances down at Pain) What.Are.Those?
Pain: (Cut to Pain, who is shown wearing a pair of plastic Air-Herc Sandals) I dunno, I thought they looked kinda... dashing.
Hades: I've got 24 hours to get rid of this... (Flares up) bozo, or the entire scheme I've been setting up for eighteen years... goes up in smoke. (Begins to burn out of control) And you... are wearing... HIS... MERCHANDIIIISE?!
Cerberus. Especially with all those TEETH. In fact, his introduction is him snarling at Hades as he's passing by.
The revelation that Hades is not like his neutral and mostly benign mythological counterpart but still rules over all the mortal souls that end up dying and essentially controls the afterlife. In the animated series, itís mentioned that Hades has no control over the Elysian Fields which Zeus reserved as a resting ground for heroes, but if youíre not a hero like Hercules then youíre in Hadesí clutches. This spells a lot of Fridge Horror for all of Hercís mortal friends and loved ones like Meg and Phil whose souls are vulnerable to eternal torture by Hades and his minions once they die.
The Cyclops and his bellowing taunting for a depowered Hercules to fight him and his eventual attempt to bite off Herculesí head: "HEERRCCUULLEESSSS!"