sea horse

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A small marine fish of the genus Hippocampus, characteristically swimming in an upright position and having a prehensile tail, a horselike head, and a body covered with bony plates.
2. n. See walrus.
3. n. Mythology An animal, half fish and half horse, ridden by Neptune and other sea gods.
4. n. A large white-capped wave.

Wiktionary

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

GNU Webster's 1913

1. 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.
2. The walrus.
3. Any fish of the genus Hippocampus.

WordNet 3.0

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