The Collaborative International Dictionary of English v.0.48:

Spavin \Spav"in\, n. [OE. spaveyne, OF. esparvain, F.
   ['e]parvin; akin to OF. espervier a sparrow hawk, F.
   ['e]pervier, fr. OHG. sparw[=a]ri (G. sperber), fr. OHG.
   sparo sparrow, because this disease makes the horse raise the
   infirm leg in the manner of a sparrow hawk or sparrow. See
   Sparrow.] (Far.)
   A disease of horses characterized by a bony swelling
   developed on the hock as the result of inflammation of the
   bones; also, the swelling itself. The resulting lameness is
   due to the inflammation, and not the bony tumor as popularly
   supposed. --Harbaugh.
   [1913 Webster]

   Bog spavin, a soft swelling produced by distention of the
      capsular ligament of the hock; -- called also blood
      spavin.

   Bone spavin, spavin attended with exostosis; ordinary
      spavin.
      [1913 Webster]

WordNet (r) 3.0 (2006):

spavin
    n 1: a swelling of the hock joint of a horse; resulting in
         lameness