The Collaborative International Dictionary of English v.0.48:

Mule \Mule\ (m[=u]l), n. [F., a she-mule, L. mula, fem. of
   mulus; cf. Gr. my`klos, mychlo`s. Cf. AS. m[=u]l, fr. L.
   mulus. Cf. Mulatto.]
   1. (Zool.) A hybrid animal; specifically, one generated
      between an ass and a mare. Sometimes the term is applied
      to the offspring of a horse and a she-ass, but that hybrid
      is more properly termed a hinny. See Hinny.
      [1913 Webster]

   Note: Mules are much used as draught animals. They are hardy,
         and proverbial for stubbornness.
         [1913 Webster]

   2. (Bot.) A plant or vegetable produced by impregnating the
      pistil of one species with the pollen or fecundating dust
      of another; -- called also hybrid.
      [1913 Webster]

   3. A very stubborn person.
      [1913 Webster]

   4. A machine, used in factories, for spinning cotton, wool,
      etc., into yarn or thread and winding it into cops; --
      called also jenny and mule-jenny.
      [1913 Webster]

   5. A slipper that has no fitting around the heel.

   Syn: mules, scuff, scuffs.
        [WordNet 1.5]

   Mule armadillo (Zool.), a long-eared armadillo (Tatusia
      hybrida), native of Buenos Ayres; -- called also mulita.
      See Illust. under Armadillo.

   Mule deer (Zool.), a large deer (Cervus macrotis syn.
      Cariacus macrotis) of the Western United States. The
      name refers to its long ears.

   Mule pulley (Mach.), an idle pulley for guiding a belt
      which transmits motion between shafts that are not
      parallel.

   Mule twist, cotton yarn in cops, as spun on a mule; -- in
      distinction from yarn spun on a throstle frame.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mule deer
    n 1: long-eared deer of western North America with two-pronged
         antlers [syn: mule deer, burro deer, Odocoileus
         hemionus]