saddle sore

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A sore on a horse's back caused by an improperly fitted saddle.
2. n. A sore on a rider caused by the chafing of a saddle.

Wiktionary

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

WordNet 3.0

1. n. an open sore on the back of a horse caused by ill-fitting or badly adjusted saddle
2. n. sore on a horseback rider chafed by a saddle