The Collaborative International Dictionary of English v.0.48:

Polecat \Pole"cat`\, n. [Probably fr. F. poule hen, and
   originally, a poultry cat, because it feeds on poultry. See
   Poultry.] (Zool.)
      (a) A small European carnivore of the Weasel family
          (Putorius f[oe]tidus). Its scent glands secrete a
          substance of an exceedingly disagreeable odor. Called
          also fitchet, foulmart, and European ferret.
      (b) The zorilla. The name is also applied to other allied
          species.
          [1913 Webster]

WordNet (r) 3.0 (2006):

polecat
    n 1: American musteline mammal typically ejecting an intensely
         malodorous fluid when startled; in some classifications put
         in a separate subfamily Mephitinae [syn: skunk,
         polecat, wood pussy]
    2: dark brown mustelid of woodlands of Eurasia that gives off an
       unpleasant odor when threatened [syn: polecat, fitch,
       foulmart, foumart, Mustela putorius]

Moby Thesaurus II by Grady Ward, 1.0:

53 Moby Thesaurus words for "polecat":
   Cape polecat, animal, ape, bar, bear, beast, cavy, chimp,
   chimpanzee, coon, cur, dog, ferret, foumart, glutton, groundhog,
   guinea pig, hedgehog, hound, hyena, insect, mongrel, monk, monkey,
   mousehound, opossum, pig, porcupine, possum, prairie dog,
   quill pig, raccoon, reptile, rotten egg, serpent, skunk, snake,
   stink ball, stink bomb, stinkard, stinker, stinkpot, swine,
   varmint, vermin, viper, weasel, whelp, whistle-pig, wolverine,
   woodchuck, worm, zoril