Source : Webster's Revised Unabridged Dictionary (1913)

Roughrider \Rough"rid`er\, n.
   One who breaks horses; especially (Mil.), a noncommissioned
   officer in the British cavalry, whose duty is to assist the
   riding master.

Roughrider \Rough"rid`er\, n.
   An officer or enlisted man in the 1st U. S. Volunteer
   Cavalry, a regiment raised for the Spanish war of 1898,
   composed mostly of Western cowboys and hunters and Eastern
   college athletes and sportsmen, largely organized, and later
   commanded, by Theodore Roosevelt. Sometimes, locally, a
   member of any of various volunteer cavalry commands raised in
   1898. [Colloq.]

Source : WordNet®

roughrider
     n : a horseman skilled at breaking wild horses to the saddle