Abstract
A five‐week‐old Arabian filly presented with a rapidly expanding congenital mass on the rostral portion of the mandible involving the deciduous incisors (#701–703 and 801–802). Radiographs revealed a mass of soft tissue and bone containing a multicameral area of marked osteolysis. On biopsy, the mass contained abundant mucinous fluid; myxosarcoma was diagnosed on clinical, histological and immunohistochemical features. Surgical excision was performed, but excision was incomplete and two months later the tumour recurred rostrally. Rostral hemimandibulectomy was performed, but the foal was euthanased five weeks after discharge because of persistent colic; no tumour recurrence or metastasis was found. Myxomatous tumours are rare in the horse, largely associated with the head and radiographic findings are consistent with a locally invasive mass on the head that appears cystic and osteolytic. While mucin within the mass is highly suggestive, histopathology is required for definitive diagnosis. Complete surgical excision with wide, clean margins is the treatment of choice.