1a small marine fish with segmented bony armour, an upright posture, a curled prehensile tail, a tubular snout, and a head and neck suggestive of a horse. The male has a brood pouch in which the eggs develop.
Genus Hippocampus, family Syngnathidae: many species, including the European H. ramulosus
2a mythical creature with a horse’s head and fish’s tail.
seahorse in other Oxford dictionaries
Definition of seahorse in the US English dictionary