Thoracic hemivertebra is one of the rare congenital vertebral anomalies in horses resulting in vertebral column deformity with or without neurological signs. A nine-month-old Racking filly was presented with a distinct bump over the back region. This bump was apparent at birth and has been increasing in size. Clinical examination revealed a painless kyphoscoliosis over the thoracic vertebrae with mild ataxia and abnormal gait of hindlimbs. In plain lateral radiographs of thoracic vertebrae, hemivertebrae were seen in the 13th-15th thoracic vertebrae. At myelography performed under general anaesthesia, the ventral and dorsal aspects of the contrast medium column were narrowing at the level of the 13th-14th thoracic vertebrae, and no sign of contrast medium after the 14th thoracic vertebra that represents spinal cord compression. Little information about hemivertebra in horses exists and a better understanding of hemivertebra etiology is required.