The sea-serpent is a monstrous finned serpent, which was believed to live in the Great Ocean and attack unwary ships. It was sometimes shown on period maps, but no examples have been found in period armory. In Society heraldry, it is most often shown “ondoyant”, or wavy, as in the illustration.
Some early Society blazons use the term “ondoyant-emergent”: wavy, couped at the waterline, with only the top portions drawn, as in the illustration. This treatment is no longer permitted in Society heraldry.
See also eel.
The Baron of Calafia bears: Azure, a sea-serpent ondoyant-emergent sable fimbriated Or, overall a trident with a laurel wreath entwined in its tines Or.
Ivan the Illustrated bears: Per fess sable and argent, a sea-serpent ondoyant within a bordure wavy counterchanged.