Hippocampus

The hippocampus or hippocamp, also hippokampos (plural: hippocampi or hippocamps; Greek: ἱππόκαμπος, from ἵππος, "horse" and κάμπος, "sea monster"), often called a sea-horse in English, is a mythological creature shared by Phoenician, Etruscan, Pictish, Roman and Greek mythology, though its name has a Greek origin. The hippocampus has typically been depicted as having the upper body of a horse with the lower body of a fish.

Mythology
Coins minted at Tyre around the 4th century BC show the patron god Melqart riding on a winged hippocampus and accompanied by dolphins. Coins of the same period from Byblos show a hippocampus diving under a galley.

A gold sea-horse was discovered in a hoard from the kingdom of Lydia in Asia minor, dating to the 6th century BC.

Greek and Roman
In the Iliad, Homer describes Poseidon, god of horses, earthquakes, and the sea, driving a chariot drawn by brazen-hoofed horses over the sea's surface, and Apollonius of Rhodes, describes the horse of Poseidon emerging from the sea and galloping across the Libyan sands. This compares to the specifically "two-hoofed" hippocampi of Gaius Valerius Flaccus in his Argonautica: "Orion when grasping his father’s reins heaves the sea with the snorting of his two-hooved horses." In Hellenistic and Roman imagery, however, Poseidon (or Roman Neptune) often drives a sea-chariot drawn by hippocampi. Thus hippocampi sport with this god in both ancient depictions and much more modern ones, such as in the waters of the 18th-century Trevi Fountain in Rome surveyed by Neptune from his niche above.

The appearance of hippocampi in both freshwater and saltwater is counter-intuitive to a modern audience, though not to an ancient one. The Greek picture of the natural hydrological cycle did not take into account the condensation of atmospheric water as rain to replenish the water table, but imagined the waters of the sea oozing back landwards through vast caverns and aquifers, rising replenished and freshened in springs.

Thus it was natural for a temple at Helike in the coastal plain of Achaea to be dedicated to Poseidon Helikonios, (the Poseidon of Helicon), the sacred spring of Boeotian Helikon. When an earthquake suddenly submerged the city, the temple's bronze Poseidon accompanied by hippocampi continued to snag fishermens' nets.Likewise, the hippocampus was considered an appropriate decoration for mosaics in Roman thermae or public baths, as at Aquae Sulis modern day Bath in Britannia.

Poseidon's horses, which were included in the elaborate sculptural program of gilt-bronze and ivory, added by a Roman client to the temple of Poseidon at Corinth, are likely to have been hippocampi; the Romanised Greek Pausanias described the rich ensemble in the later 2nd century AD (Geography of Greece ii.1.7-.8): On the temple, which is not very large, stand bronze Tritons. In the fore-temple are images, two of Poseidon, a third of Amphitrite, and a Sea, which also is of bronze. The offerings inside were dedicated in our time by Herodes Atticus, four horses, gilded except for the hoofs, which are of ivory, and two gold Tritons beside the horses, with the parts below the waist of ivory. On the car stand Amphitrite and Poseidon, and there is the boy Palaemon upright upon a dolphin. These too are made of ivory and gold. On the middle of the base on which the car has been wrought a Sea holding up the young Aphrodite, and on either side are the nymphs called Nereids.