saddle sore

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

• noun A sore on a horse's back caused by an improperly fitted saddle.
• noun A sore on a rider caused by the chafing of a saddle.

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

• noun A gall, a sore developed by a horse from an ill-fitting or ill-adjusted saddle.
• adjective Pertaining to a horse with saddle sores, or to a rider who has been riding too long.

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

• noun an open sore on the back of a horse caused by ill-fitting or badly adjusted saddle
• noun sore on a horseback rider chafed by a saddle