The hippocamp or hippocampus (plural: hippocampi; Greek: ἵπποκαμπος, from ἵππος, "horse" and κάμπος , "monster), often called a sea-horse in English, is a mythological creature shared by Phoenician and Greek mythology, though the name by which it is recognized is purely Greek; it became part of Etruscan mythology. It has typically been depicted as a horse in its forepart with a coiling, scaly, fishlike hindquarter.
The appearance of hippocamps 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 account of the condensation of atmospheric water as rain to replenish the water table, but imagined the refreshening of the waters of the sea oozing back landwards through vast underground 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 hippocamps continued to snag fishermen's nets.
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 hippocamps; the Romanized Greek Pausanias described the rich ensemble in the later second century CE (Geography of Greece ii.1.7-.8):
Hippocamps appear with the first Orientalizing phase of Etruscan civilization: they remain a theme in Etruscan tomb wall-paintings and reliefs, where they are sometimes provided with wings, as they are in the Trevi fountain. Katharine Shepard found in the theme an Etruscan belief in a sea-voyage to the other world
Aside from aigikampoi, the fish-tailed goats representing Capricorn or Aegeus ("goat-man") other fish-tailed animals rarely appearing in Greek art but more characteristic of the Etruscans included leokampoi (fish-tailed lions), taurokampoi (fish-tailed bulls) or pardalokampoi (fish-tailed leopards).
The mythic hippocamp has been used as a heraldic charge, particularly since the Renaissance, most often in the armorial bearings of people and places with maritime associations. However, in a blazon, the terms hippocamp and hippocampus now refer to the real animal called a seahorse, and the terms seahorse and sea-horse refer to the mythological creature. The above-mentioned fish hybrids are seen less frequently.
In modern fantasy, this creature is a part of various milieus, including the stories of Harry Potter and the many fictional worlds of Dungeons & Dragons. In both these examples, the Latin hippocampus is the form of the name which is used.
The Dungeons & Dragons context provides the following specific description:
The hippocampus is the most prized of the marine steeds, a creature that combines features of a horse and a fish. The hippocampus has the head, forelegs, and torso of a horse. The equine section is covered with short hair. The mane is made of long, flexible fins. The front hooves are replaced by webbed fins that fold up as the leg moves forward, then fan out as the leg strokes back. Past the rib cage the body becomes fish-like. The tail tapers 14 feet into a wide horizontal fin. A dorsal fin is located on the rump. Coloration is that of seawater. Typical colors include ivory, pale green, pale blue, aqua, deep blue, and deep green."
The videogame Age of Mythology also uses the Hippocamp as a player controlled unit used for the exploration of the sea, available only to players who chose to worship Poseidon.