A 13-year-old cob mare was presented with exophthalmos and periocular swelling of the left eye. The diagnostic work-up included ocular ultrasound, sonographic examination through the thinned frontal bone, radiography, standing computed tomography of the skull and exploratory osteoplastic surgery. Histopathology was consistent with an organized hematoma. An intradiploic hematoma of the frontal bone was diagnosed 5 years after head trauma, with progressive expansion and deformation of the skull resulting in exophthalmos. Exophthalmos with facial bone deformation was the only clinical finding of intradiploic hematoma. Standing computed tomography (CT) aided the diagnosis to differentiate intradiploic hematoma from other, more common causes of facial bone distortion associated with paranasal sinus diseases. Intradiploic hematoma of possible traumatic origin is a differential diagnosis for sinonasal disease and exophthalmos in the horse.