The term 'UNBROKEN Mustang' refers to a wild horse that has never been tamed. The two crossword solutions, BRONCO (6 letters) and BRONC (5 letters), are both terms used to describe a wild horse, typically found in the western United States. The term 'BRONCO' is derived from the Spanish word 'BRONCO, ' which means 'ROUGH' or 'UNBROKEN.' The term 'BRONC' is a shortened version of 'BRONCO.' Both terms refer to an untamed horse that has not been broken in for riding.