The Collaborative International Dictionary of English v.0.48:

Armadillo \Ar`ma*dil"lo\ ([aum]r`m[.a]*d[i^]l"l[-o]), n.; pl.
   Armadillos (-l[=o]z). [Sp. armadillo, dim. of armado armed,
   p. p. of armar to arm. So called from being armed with a bony
   shell.] (Zool.)
      (a) Any edentate animal if the family Dasypid[ae],
          peculiar to America. The body and head are incased in
          an armor composed of small bony plates. The armadillos
          burrow in the earth, seldom going abroad except at
          night. When attacked, they curl up into a ball,
          presenting the armor on all sides. Their flesh is good
          food. There are several species, one of which (the
          peba) is found as far north as Texas. See Peba,
          Poyou, Tatouay.
      (b) A genus of small isopod Crustacea that can roll
          themselves into a ball.
          [1913 Webster]

WordNet (r) 3.0 (2006):

armadillo
    n 1: burrowing chiefly nocturnal mammal with body covered with
         strong horny plates

Moby Thesaurus II by Grady Ward, 1.0:

258 Moby Thesaurus words for "armadillo":
   Angora goat, Arctic fox, Belgian hare, Caffre cat, Indian buffalo,
   Kodiak bear, Virginia deer, aardvark, aardwolf, alpaca, anteater,
   antelope, antelope chipmunk, aoudad, apar, 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, platypus,
   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