sea cow

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of several large, cylindrical, herbivorous marine mammals of the order Sirenia, having a paddlelike tail and rounded front flippers, including the manatee, dugong, and the very large species Hydrodamalis stelleri of the northern Pacific that became extinct in the late 1700s. Also called sirenian.

Century Dictionary and Cyclopedia

1. n. The walrus. Also sea-ox, sea-horse.
2. n. A lately extinct sirenian of the North Pacific, Rhytina stelleri: more fully called arctic, northern, or Steller's sea-cow. See Rhytina.
3. n. Any sirenian, as the manatee, dugong, or halicore.
4. n. The hippopotamus: translating a name of the Dutch colonists.
5. n. The sea-cucumber, Holothuria nigra.

Wiktionary

1. n. Any of several marine mammals of the order Sirenia, including the manatee and dugong.

GNU Webster's 1913

1. The mantee.
2. The dugong.
3. The walrus.

WordNet 3.0

1. n. any of two families of large herbivorous aquatic mammals with paddle-shaped tails and flipper-like forelimbs and no hind limbs