Source : Webster's Revised Unabridged Dictionary (1913)

Horsehair \Horse"hair`\, n.
   A hair of a horse, especially one from the mane or tail; the
   hairs of the mane or tail taken collectively; a fabric or
   tuft made of such hairs.

   {Horsehair worm} (Zo["o]l.), the hair worm or gordius.

Source : WordNet®

horsehair
     n 1: hair taken from the mane or tail of a horse
     2: fabric made from horsehair fibers; used for upholstery