The Sea-horse is of very frequent use in armory, and usually has reference to meritorious actions performed at sea. It is also borne by many seaport towns in allusion to the trade and commerce of the port, as in the arms of the city of Belfast.

From Fictitious and Symbolic Creatures in Art by John Vinycomb, p273

This fabulous marine creature [the sea-horse] in heraldry is compounded of the fore quarters of a horse with webbed paws, and the hinder part of a fish or dolphin. A scalloped fin is continued down the neck and back in place of a mane. It is frequently, though erroneously, to be seen depicted with the flowing mane of a horse.

From Fictitious and Symbolic Creatures in Art by John Vinycomb, p272