sea horse

from the GNU version of the Collaborative International Dictionary of English.

• A fabulous creature, half horse and half fish, represented in classic mythology as driven by sea dogs or ridden by the Nereids. It is also depicted in heraldry. See hippocampus.
• The walrus.
• Any fish of the genus Hippocampus.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any of various small marine fish of the genus Hippocampus that have a horselike head and swim upright.
• noun The walrus.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun small fish with horse-like heads bent sharply downward and curled tails; swim in upright position
• noun either of two large northern marine mammals having ivory tusks and tough hide over thick blubber