A dentigerous cyst (Latin: cystisdentigerus), also known as a follicular cyst, is a pathological condition noted in both medical and veterinary surgical practice. Clinical symptoms usually appear in patients before reaching maturity, although there are exceptions. The main thesis concerning the origin of this phenomenon indicates developmental disorders at the prenatal stage of organogenesis. The tooth bud tissue of the branchial arch shifts towards the temporal, frontal, ethmoid and maxillary bones. The presence of a cyst, or rather a tooth bud, or tooth-like growths in its vicinity can present a number of nonspecific pathological symptoms, depending on its location. These include the deformation and asymmetry of the cranial bones, sinusitis, secondary bacterial infections, and head-shaking syndrome. Diagnostic methods used in equine medicine include radiology, CT, and endoscopic examinations. Field practice has certain limitations. For example, it does not allow for the use of computed tomography, which does not preclude the performance of even complicated surgical procedures. The present article is an example of this. The mare underwent a clinical examination during which a regular, fluctuating swelling was diagnosed, extending from the base of the right auricle to the fronto-zygomatic region. The lesion was incised, the fluid inside was drained, and its interior was then palpated. A fistulous opening of about 4 cm in diameter was diagnosed in the frontal bone at the suture with the zygomatic bone. At the bottom of the fistula there was pathological tissue resembling the grinding surface of a cheek tooth. Detailed examinations were carried out using X-ray and an endoscope to plan the surgery to remove the pathological lesion: the tooth-like tissue from the frontal sinus. Due to the fact that the procedure was planned to be performed in a stable, the concept of sinus trephination was rejected and it was decided that surgical access would be through the cyst opening. The clinical examination of the described mare’s oral cavity showed no clinical changes and the dental formula was characteristic of her age. The present article describes a case, unprecedented in literature, of the presence of a tooth-like tissue located at the border of the frontal sinus and the ethmoid bone. The treatment required surgical intervention, i.e. an osteotomy using orthopaedic drill bits, resulting in the patient regaining full function.