The Collaborative International Dictionary of English v.0.48:

Muskrat \Musk"rat`\, n.
   1. (Zool.) A North American aquatic fur-bearing rodent
      (Ondatra zibethica formerly Fiber zibethicus). It
      resembles a rat in color and having a long scaly tail, but
      the tail is compressed, the hind feet are webbed, and the
      ears are concealed in the fur. It has scent glands which
      secrete a substance having a strong odor of musk. Called
      also musquash, musk beaver, ondatra, and sometimes
      water rat.
      [1913 Webster +PJC]

   2. (Zool.) The musk shrew.
      [1913 Webster]

   3. (Zool.) The desman.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Desman \Des"man\ (d[e^]s"man), n. [Cf. Sw. desman musk.] (Zool.)
   An amphibious, insectivorous mammal found in Russia (Myogale
   moschata). It is allied to the moles, but is called
   muskrat by some English writers. [Written also
   d[ae]sman.] Desmid

WordNet (r) 3.0 (2006):

muskrat
    n 1: the brown fur of a muskrat [syn: muskrat, muskrat fur]
    2: beaver-like aquatic rodent of North America with dark glossy
       brown fur [syn: muskrat, musquash, Ondatra zibethica]