They were the fish-tailed horses of the sea. They were depicted as composite creatures with the head and fore-parts of a horse and the serpentine-tail of a fish.
The ancients believed they were the adult-form of the small fish we call the “sea-horse”. Hippocampi were the mounts of Nereid nymphs and sea-gods, and Poseidon drove a chariot drawn by two or four of the creatures.
Other fish-tailed land animals which appear in ancient art include the Leocampus (fish-tailed lion), Taurocampus (fish-tailed bull), Pardalocampus (fish-tailed leopard), and Aegaecampcs (fish-tailed goat).
Digital Maps of the Ancient World 