sea cow

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of several large herbivorous marine mammals of the order Sirenia, having a paddlelike tail, forelimbs shaped like flippers, and no hind limbs, and including the manatee, the dugong, and Steller's sea cow (Hydrodamalis stelleri) of the northern Pacific, which became extinct in the late 1700s.

from The Century Dictionary.

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

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

• The mantee.
• The dugong.
• The walrus.

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

• noun Any of several marine mammals of the order Sirenia, including the manatee and dugong.

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

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