roughrider

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A skilled rider of little-trained horses, especially one who breaks horses for riding.
• noun A member of the First US Volunteer Cavalry regiment under Theodore Roosevelt in the Spanish-American War.

from The Century Dictionary.

• noun One who breaks young or wild horses to the saddle; in the British army, a non-commissioned cavalry or artillery officer detailed to assist the riding-master, one being allowed to each troop or battery.
• noun A horseman accustomed to rough or hard riding, such as cow-boys or frontiersmen: applied specifically, in the late Spanish-American war, to the members of a volunteer regiment of cavalry, recruited partly from Western cow-boys.
• noun One who breaks young or wild horses to the saddle; in the army, a non-commissioned cavalry or artillery officer detailed to assist the riding-master, one being allowed to each troop or battery.
• noun Loosely, a horseman occupied with hard, rough work.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who breaks horses; especially (Mil.), a noncommissioned officer in the British cavalry, whose duty is to assist the riding master.
• noun colloq. 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A horsebreaker.
• noun military, dated A noncommissioned officer in the British cavalry whose duty is to assist the riding master.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a member of the volunteer cavalry regiment led by Theodore Roosevelt in the Spanish-American War (1898)
• noun a horseman skilled at breaking wild horses to the saddle