We present a case of unilateral internal carotid artery (ICA) agenesis with trans-sellar anastomosis associated with a right aortic arch, and discuss the embryonic development and clinical implications of this condition. A 26-year-old woman without significant medical history was referred to our hospital complaining of tinnitus. She denied any other symptoms, and examination did not reveal any other neurological deficits. Radiological examinations showed agenesis of the right ICA with trans-sellar anastomosis, and a right aortic arch. This vascular configuration reflects the anatomical homology with avian vascular structure. Recognition of this anomaly has important implications for clinical situations, such as the planning of carotid or transsphenoidal surgery, the management of thromboembolic disease, and the surveillance and detection of associated cerebral aneurysms.