The Collaborative International Dictionary of English v.0.48:

Platypus \Plat"y*pus\, n. [NL., fr. Gr. ? + ? foot.] (Zool.)
   The duck mole. See under Duck.

   Syn: Duck Mole, duckbill, duck-bill platypus, duck-billed
        platypus. [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Duck \Duck\, n. [OE. duke, doke. See Duck, v. t. ]
   1. (Zool.) Any bird of the subfamily Anatin[ae], family
      Anatid[ae].
      [1913 Webster]

   Note: The genera and species are numerous. They are divided
         into river ducks and sea ducks. Among the former
         are the common domestic duck (Anas boschas); the wood
         duck (Aix sponsa); the beautiful mandarin duck of
         China (Dendronessa galeriliculata); the Muscovy duck,
         originally of South America (Cairina moschata). Among
         the sea ducks are the eider, canvasback, scoter, etc.
         [1913 Webster]

   2. A sudden inclination of the bead or dropping of the
      person, resembling the motion of a duck in water.
      [1913 Webster]

            Here be, without duck or nod,
            Other trippings to be trod.           --Milton.
      [1913 Webster]

   Bombay duck (Zool.), a fish. See Bummalo.

   Buffel duck, Spirit duck. See Buffel duck.

   Duck ant (Zool.), a species of white ant in Jamaica which
      builds large nests in trees.

   Duck barnacle. (Zool.) See Goose barnacle.

   Duck hawk. (Zool.)
      (a) In the United States: The peregrine falcon.
      (b) In England: The marsh harrier or moor buzzard.

   Duck mole (Zool.), a small aquatic mammal of Australia,
      having webbed feet and a bill resembling that of a duck
      (Ornithorhynchus anatinus). It belongs the subclass
      Monotremata and is remarkable for laying eggs like a bird
      or reptile; -- called also duckbill, platypus,
      mallangong, mullingong, tambreet, and water mole.


   To make ducks and drakes, to throw a flat stone obliquely,
      so as to make it rebound repeatedly from the surface of
      the water, raising a succession of jets; hence:

   To play at ducks and drakes, with property, to throw it
      away heedlessly or squander it foolishly and unprofitably.


   Lame duck. See under Lame.
      [1913 Webster]

WordNet (r) 3.0 (2006):

platypus
    n 1: small densely furred aquatic monotreme of Australia and
         Tasmania having a broad bill and tail and webbed feet; only
         species in the family Ornithorhynchidae [syn: platypus,
         duckbill, duckbilled platypus, duck-billed platypus,
         Ornithorhynchus anatinus]

Moby Thesaurus II by Grady Ward, 1.0:

258 Moby Thesaurus words for "platypus":
   Angora goat, Arctic fox, Belgian hare, Caffre cat, Indian buffalo,
   Kodiak bear, Virginia deer, aardvark, aardwolf, alpaca, anteater,
   antelope, antelope chipmunk, aoudad, apar, armadillo, ass, aurochs,
   badger, bandicoot, bassarisk, bat, bear, beaver, bettong,
   binturong, bison, black bear, black buck, black cat, black fox,
   black sheep, blue fox, bobcat, brown bear, brush deer, brush wolf,
   buffalo, buffalo wolf, burro, burro deer, cachalot, camel,
   camelopard, capybara, carabao, caribou, carpincho, cat,
   cat-a-mountain, catamount, cattalo, cavy, chamois, cheetah,
   chevrotain, chinchilla, chipmunk, cinnamon bear, coon, coon cat,
   cotton mouse, cotton rat, cougar, cow, coyote, coypu, deer,
   deer tiger, dingo, dog, donkey, dormouse, dromedary, echidna,
   eland, elephant, elk, ermine, eyra, fallow deer, ferret,
   field mouse, fisher, fitch, flying phalanger, foumart, fox,
   fox squirrel, gazelle, gemsbok, genet, giraffe, glutton, gnu,
   gnu goat, goat, goat antelope, gopher, grizzly bear,
   ground squirrel, groundhog, guanaco, guinea pig, hamster, hare,
   harnessed antelope, hartebeest, hedgehog, hippopotamus, hog, horse,
   hyena, hyrax, ibex, jackal, jackass, jackrabbit, jaguar,
   jaguarundi, jerboa, jerboa kangaroo, kaama, kangaroo,
   kangaroo mouse, kangaroo rat, karakul, kinkajou, kit fox, koala,
   lapin, lemming, leopard, leopard cat, lion, llama, lynx, mammoth,
   mara, marmot, marten, mastodon, meerkat, mink, mole, mongoose,
   moose, mouflon, mountain goat, mountain lion, mountain sheep,
   mouse, mule, mule deer, muntjac, musk deer, musk hog, musk-ox,
   muskrat, musquash, nilgai, nutria, ocelot, okapi, onager, oont,
   opossum, otter, ounce, ox, pack rat, painter, panda, pangolin,
   panther, peccary, peludo, phalanger, pig, pine mouse,
   pocket gopher, pocket mouse, pocket rat, polar bear, polar fox,
   polecat, porcupine, possum, pouched rat, poyou, prairie dog,
   prairie wolf, pronghorn, puma, rabbit, raccoon, rat, red deer,
   red squirrel, reindeer, rhinoceros, roe, roe deer, roebuck, sable,
   serval, sheep, shrew, shrew mole, sika, silver fox, skunk, sloth,
   snowshoe rabbit, springbok, squirrel, stoat, suslik, swamp rabbit,
   swine, takin, tamandua, tamarin, tapir, tarpan, tatou, tatou peba,
   tatouay, tiger, tiger cat, timber wolf, tree shrew, urus, vole,
   wallaby, warthog, water buffalo, waterbuck, weasel, wharf rat,
   whistler, white fox, wild ass, wild boar, wild goat, wild ox,
   wildcat, wildebeest, wolf, wolverine, wombat, wood rat, woodchuck,
   woolly mammoth, yak, zebra, zebu, zoril