Classical Mythology/Nightmare Fuel

Monsters like the Hydra (a serpent with nine heads, that can continually regenerate when injured), the Chimera (a hideous amalgamation of a lion, a goat and a dragon), Cerberus (a vicious three-headed dog that in some myths had a back covered in living serpents), Orthrus (Cerberus's two-headed, serpent-tailed little brother), and various other creatures definitely count. Perhaps the worst is Typhon -- father of the above along with the Mother of All Monsters, Echidna -- Typhn is described by some writers as being as tall as the sky itself, and having a hundred dragon-like heads, all of which screamed and breathed fire. It's not hard to see why almost all the gods had a collective Brown Note when he appeared, and fled Greece, leaving Zeus to face the creature by himself.
- And then there's creatures like Scylla and, even worse, Charybdis, once beautiful women turned into eldritch things of pure horror. Scylla we at least know is horrifying to observe, looking like a giant, beautiful women from the waist up, with a scaled tail below, and the heads of six, rapid wolves snapping at her waist.
Prometheus' fate of being chained to a rock and having an eagle peck out his liver each day. He endured it for hundreds of years before being released.
- Similarly the fate Cheiron the centaur faced before he sold his immortality: living forever, with the maddening poison of the Hydra eternally burning through his veins.
The fate of mortals unfortunate enough to piss off the Olympians.
- Actaeon, a man who accidentally walked in on a bathing Artemis got himself turned into a deer and killed by his own dogs.
- A man who chopped down one of Demeter's trees was infected with eternal hunger and eventually ate himself.
The few tales of cannibalism (such as Tantalus and Atreus).
Some of the bandits whom Theseus defeated were known for their exceedingly barbaric cruelty such as Procrustes, who tied his victims to a bed and chopped body parts if they did not fit. Sinis was also pretty horrific: he bent down two pine trees with his great strength, tied the hands of passer-bys to the tips of the two trees, and then let go.
The minotaur's young victims were imprisoned in the labyrinth and force to run for their lives until the beast catches and devours them or they drop dead of exhaustion, hunger and thirst. Its like the first teen slasher flick.
The future Olympians were able to live and grow in Kronos's stomach how exactly? Squick indeed.
Antaeus the giant who was building a temple using human skulls.
The fate of the few mortals sent to Tartarus
- Ixion - bound to an ever spinning wheel.
- Sisyphus - forced to roll a boulder up a hill only for it to roll back down.
- Tantalus - chained to a pool from which he can never drink because the water recedes. There are fruit trees whose branches he can't reach because the breeze blows them away.
- On the other hand, all three had it coming: Tantalus butchered and cooked his own child and tried to feed him to the Gods as a sacrifice (Demeter actually ate a piece by accident).