Argus
The one that sees all. A man with many eyes. Early accounts say he had four eyes, later a hundred. There are many accounts of his parentage. He was in a number of adventures: He killed a bull ravaging Arcadia, he killed a satyr for stealing cattle, he killed Echidna, he avenged the death of Apis.
Argus was killed by Hermes while he guarded Io for Hera.
Chimaera
Spawned by Typhoeus and Echidna. Its body was mixed having the front part of a lion, middle of a goat, and snake for a tail. It breathed fire. It ravaged Lycia, killing cattle and setting fires until it was killed by Bellerophon.
Cyclopes
The Cyclopes were gigantic one eyed monsters. There were three of them who represented thunder, lightning, and the lighting bolt. They are named Brontes, Steropes, and Arges. They were born to Gaea and Uranus. They were also the first smiths. When Cronus came to power he imprisoned the Cyclopes in Tartarus. They were released by Zeus and fought with him against the Titans. As a reward for their release, the Cyclopes gave Zeus his weapons of lighting and thunder. They continued as his workers at Mount Olympus forging his thunderbolts.
Apollo killed at least one of the Cyclopes in retribution for Zeus killing his son Asclepius.
Echidna
A female monster consisting of half nymph, half speckled snake. It lived in a cave where it came out to snatch up and eat those passing. The critter was ageless but, not immortal. It was killed by Argus while sleeping. It mated frequently with Typhoeus, producing a variety of offspring.
Hecatoncheires
Hecatoncheires means "hundred-handed". They were gigantic and had fifty heads and one hundred arms each of great strength. There were three of them: Briareus also called Aegaeon, Cottus, and Gyges also called Gyes. They were born to Gaea and Uranus. Their mutual hatred of Uranus caused him to force the Hecatoncheires back into Gaea's womb. This precipitated Gaea's rebellion against Uranus. When Cronus came to power, he imprisoned them in Tartarus. They were released by Zeus and fought with him against the Titans. They were able to hurl huge boulders as many as a hundred at a time against their opponents. One of them, Briareus, served as Zeus's bodyguard.
Giants
The Giants were created from Uranus blood as a result of his castration by Cronus. They became powerful enough to try to unseat Zeus and the Olympians. The gods needed the help of a mortal in order to kill the Giants. Hercules was the one who helped the gods beat down the Giants. When the gods won, they imprisoned the Giants in Tartarus.
Python
Python was a monsterous snake or dragon which Apollo had to kill before he founded his oracle at Delphi. This act symbolized the truimph of the Olympian gods over earlier underworld gods. Because Python was immortal, Apollo had to do penance for the killing. For this, he founded the Pythian games in Python's honor.
Gorgons
These were female monsters with snakes for hair. Their faces are so ugly that any man who saw their faces will turn to stone. Oddly, the three Gorgons have very different origins. Stheno and Euryale were born immortal from Phorcys and Ceto. Medusa was not.
Medusa
Medusa began life as a girl so lovely that she made Athena jealous. Such beauty turned Poseidon's head. Unfortunately, he seduced the girl in one of Athena's temples. Such an affront to Athena would not go unpunished particularly by one already in ill favor. So Medusa was turned by Athena into a Gorgon. Since she started as human she remained mortal. As if this were not punishment enough, Athena later helped Perseus hunt her down and kill her.
When Perseus, cut her head off Pegasus and Chrysaor, her children by Poseidon, sprang forth. Drops of her blood fell into the Libyan desert and turned into snakes. Later, one of these snakes killed Mopsus. After Perseus left with her head Athena took charge of what was left of her body. She flayed off Medusa's skin and made it into her trademark Aegis. She took two drops of the blood to King Erichthonius explaining that one would cure disease and the other was a deadly poison.
Ash Tree Nymphs
The Ash Tree Nymphs were created from Uranus' blood as a result of his castration by Cronus.
Typheous
Typhoeus (Typhoon), was a fire breathing dragon with a hundred heads that never rested. As a last effort, Gaea gave birth to Typhoeus to prevent Olympians from gaining power over her children the Titans. It came close to succeeding, setting most of the gods to flight and capturing Zeus. Hermes was able to free Zeus. Zeus was then able to dispatch Typhoeus with his lightning bolts. Typhoeus was buried under Mount Etna in Sicily.
Cerberus
Cerberus was one of the offspring of Typhoeus and Echidna. It was a three headed dog with a snake tail and snake heads proturding from its back. He guarded the entrance to the underworld, allowing the dead to enter but never to leave. One of the few living mortals who got past Cerberus was Orpheus who charmed it to sleep with his song during his attempt to rescue Eurydice from death. Fetching Cerberus from the underworld and displaying him to King Eurystheus was the last labor of Hercules.
Sirens
The Sirens are sisters who lure sailors to their death. The song of the Sirens is irresistible. But, they resided beyond impassable reefs which destroyed the sailor's boat when they try to reach the Sirens. Among those tempted were Jason on the Argo and Odysseus.
Pegasus
Pegasus was a winged horse and a good flyer. Pegasus was born as a result of the ill fated mating of Medusa and Poseidon. It was born from Medusa when her head was cut off by Perseus. Tamed by Bellerophon, it served as his mount during his adventures which included his slaying of the Chimaera. When Bellerophon attempted to fly Pegasus to Mount Olympus, he was dismounted by Zeus. Pegasus continued on and made it to Mount Olympus. Here, Pegasus spent his days carrying lighting bolts for Zeus.
Chrysaor
Chrysaor was born as a result of the ill fated mating of Medusa and Poseidon. He was born from Medusa when her head was cut off by Perseus. Little is known of Chrysaor other then he was considered a stout hearted warrior. His name meant Golden Sword. He fathered Geryon. His appearance was unknown but, given his family it was likely to have been unusual. He was possibly a giant.