A two-year-old Standardbred colt was referred to Cornell’s Equine Surgery Service for a right-sided tie-back procedure to treat the patient’s inspiratory stridor and exercise intolerance. Ancillary procedures such as resting and dynamic endoscopic examination of the larynx as well as trans-esophageal and percutaneous ultrasound examination of the larynx were performed in addition to the physical exam. Based on the findings the patient was diagnosed with a fourth branchial arch defect. The pathogenesis, clinical findings, treatments and prognosis for fourth branchial arch defect in horses is described.