Gorgons
Composed of three frightful sisters, namely: Stheno ("the mighty"); Euryale ("the far-springer"); and the best known and the only mortal among them, Medusa ("the queen"). They had glaring eyes, sharp fangs, brazen claws and hair of living, poisonous snakes; and anyone who glanced at them were turned to stone by their direct gaze. They figure mainly in the story of Perseus, who, with the aid of Athena and Hermes, was able to defeat Medusa by cutting off her head while looking at her reflection on a shield.
Pegasus
The majestic winged stallion that sprang from the blood from the decapitated head of Medusa. Pegasus created Hippocrene, a spring sacred to the Muses, when he struck his hoof on Mount Helicon. He was also the carrier of the Zeus' thunderbolts on Mount Olympus.
Chimera
Fire-breathing she-monster, generally represented with a lion's head, a goat's body and a dragon's tail. Its sighting was taken to be an omen of shipwrecks, storms and natural disasters particularly volcanic eruptions. The Greek hero Bellerophon was able to defeat it with the help of the winged horse Pegasus by running through it with a spear.
Centaur
A race of creatures that had the head and torso of a human being, and the lower body and legs of a horse. They were typically depicted as wild, violent and barbaric beings, indulgent of their own animal desires. A notable exception would be the intelligent, kind and civilized Chiron who tutored Theseus, Achilles and Heracles and was greatly revered as a teacher.
Minotaur
Monster with a human body with bull's head and tail. It was born to Pasiphae, the queen of Crete, after she mated with a sacred bull. It dwelt in the labyrinth, which was a complicated maze constructed by Daedalus as commanded by King Minos of Crete. Seven young men and seven maidens were sent from Athens annually to be its prey. To stop the slaughter, Theseus volunteered to fight the Minotaur, and was able to slay the monster with the help of King Minos' daughter Ariadne who gave him a magic sword and a ball of thread that allowed him to retrace his way out of the labyrinth.
Sphinx
A horrific monster, often depicted as a creature with the head and breast of a human female and the body of a lion with wings of an eagle. In the story of Oedipus, it sat on a high rock outside Thebes and accosted all passersby, carrying away and devouring anyone who could not answer its riddle: "What creatures walk on four legs in the morning, on two at noon, and on three in the evening?" Oedipus was able to answer correctly -- human beings, who crawl on all four as infants, walk upright in maturity and use a walking stick in old age. It then threw itself from the high rock and died.
Geryon
A winged giant with three human bodies conjoined at the waist. He lived on the island of Erytheia and owned a herd of cattle that was guarded by his two-headed dog Orthrus, and a man named Eurythion. For his tenth labor, Heracles was to obtain this giant's cattle; and succeeded in his task slaying Orthrus, Eurythion, and eventually the giant, tearing his body into three pieces.
Cerberus
The monstrous three-headed dog guarding the gate of Hades or the underworld to make sure that no dead could leave and no living could enter. But this brother of Orthrus was overcome several times when Orpheus used his beautiful singing voice to lull him to sleep in an effort to save his beloved Eurydice; and when Heracles, for his final labor, was able to capture this vicious hound by treating it with the first kindness it had ever received.
Hecatonchires
A trio of giants of incredible strength and ferociousness, consisting of Briareus ("The Vigorous"), Cottus ("The Furious") and Gyges ("The Big-Limbed"). They had fifty heads and a hundred arms. In fact, the name means "The hundred handed." Their father Uranus, who saw them as monsters, cast them into Tartarus (underworld). But Cronus, another son of Uranus, rescued them to help him overthrow their father. Subsequently, they would become the guards of the gates of Tartarus.
Cyclops
A race of giants that had a single eye in the middle of their forehead. They were great weapon makers who forged thunderbolts that Zeus used to overthrow Cronus and the Titans. In Homer's "Odyssey," Polyphemus was one such giant whose only eye was pierced with a hardened spear by Odysseus and his men.
Hydra
A water serpent which had breath that's lethal to both man and beast, and possessed nine heads, eight of which were mortal, while the remaining middle one was immortal. Its lair was the lake of Lerna in the Argolid where it guarded an entrance to the underworld lying beneath its waters. Heracles was tasked to kill this serpent for his second labor, but everytime he cut off each head, two new heads would sprout in its place. So he sent for the assistance of his nephew Iolaus who would burn the neck stumps with a torch to prevent new heads from growing. He then ripped off the immortal head burying it deep in the ground, and placed a huge rock on it.
