Almost all the antagonistic monsters in Greek legend were spawned from the lineage of Pontus, the sea, and Tartarus, the depths. As such, many of them represent aspects of the ocean.
Pontus mated with Gaia, the earth mother. His offspring included Nereus, Eurybia, Thaumus, Phorcus and Ceto.
The Harpies were born from the marriage of Electra, an oceanid, and Thaumus. Earlier stories depicted them as beautiful women with feathered wings, like their sister Iris. Later they became horrid monsters; the body and wings of a bird and the torso and head of a woman. Their faces were pale and ugly and their talons were vicious.
They were the Snatchers, the hounds of Zeus; they acted as beings of punishment. They stole food and stole people, who were never seen again. They tormented the blind king Phineus when he abandoned his wife Cleopatra. King Pandone had incurred the wrath of the gods - so the harpies snatched his daughers Clytia and Cameira and took them to the Eumenides to be tortured.
Aello, Celaeno and Ocypete ('Storms', 'Black Clouds' and 'Swift Flier') were harpies of note. They caused wind storms and rough seas. Another harpy, Podarge, mated with the wind god Zephyros to produce the two immortal horses Xanthos and Balios.
Medusa, Stheno and Euryale. They were the daughers of Phorcus and Ceto. Those who gazed upon their hideous faces were turned to stone. Each sister had snake entwined hair, intense eyes, tusks and course beards. Their claws were bronze and they had golden wings.
Their home was in the utmost place towards night, beyond the stream of Oceanos, by the singing Hesperides.
Medusa was once beautiful, with the loveliest hair of the three sisters. (story about how she got snakey hair)
Medusa was mortal, her two sisters were ageless. She was beheaded by Perseus on his quest. Upon her death, from her blood and Posiedons seed, both the winged horse Pegasus and the giant Khrysaor were born. The head of Medusa held the power of petrification even after death. Perseus utilised this many times, turning Atlas into a mountain and seaweed into coral.
The grey sisters: Pemphrido, Enyo and Deino ('Alarm', 'Horror' and 'Dread'); they were the white foam of ocean waves - daughters of Phorcus and Ceto and the sisters and guardians of the Gorgons. Pemphrido was robed in beauty, Enyo robed in saffron. All three were shaped as swans. They were grey from birth, and although immortal, grew older until they only had one eye and one tooth to share between them. Perseus tricked them and 'borrowed' their eye during his quest to slay Medusa. This never-sleeping eye was finally thrown into lake Triton.
A dragon, an offspring of Phorcus and Ceto. The youngest of the deadly snakes. With the Hesperides, Ladon guarded the golden apples in Titan's garden. Despite being huge and having one hundred heads, Ladon was defeated by Heracles.
The divine and haughty Echidna, the she-viper, the daughter of Phorcus and Ceto. She had the body of a serpent and black eyes but was otherwise a beautiful nymph. She lived in a cave in the desert of the Arimoi with her lover Typhon, the lawless and violent son of Tartarus. After Typhon's failed uprising against the Olympians, Echidna was permitted to bear monsters as challenges for mortal heroes1.She met her end when Argus killed her as she slept.
Her children included Cerberus, Chimera and Orthos.
The hound of Hades, a three-headed dog with lion's claws, serpents tail and a mane of snakes. It guarded the gates to the underworld, leashed on a chain of adamant. It's favoured tactic was to allow mortals to enter, but then eat them as they tried to leave. Either way it could be pacified by throwing a cake to it. Cerberus was defeated by Heracles during his final labour.
The brother of Cerberus, Orthos was a two-headed hound with a serpents tail. He was the herding dog of Eurytion, guarding the crimson cattle of Geryon. Both Orthos and Eurytion were clubbed to death by Heracles.
The fire-breathing monster of Lycia. She was the union of three animals: The fore-parts of a strong lion, the swift-footed hindquarters of a goat, and the tail of a serpent. She had three heads, one for each animal form.
Bellerophon, mounted on Pegasus, killed Chimera by driving a lance tipped with lead into the mouth of her goat head. This throat was the source of her firey breath - the lead melted and suffocated her.
She was the lover of Orthos and mothered the Sphinx and the Nemeian Lion.