Six young horses presented with a rapidly expanding maxillary mass with concurrent nasal discharge. All horses had a nonpainful firm unilateral facial swelling centred over the maxillary sinuses, accompanied by mucopurulent nasal discharge. Diagnostic imaging revealed an infiltrative nasomaxillary mass with cheek tooth involvement, diagnosed as odontogenic myxoma. The tooth involvement included missing, malformed and/or displaced dental precursors or unerupted teeth. Due to the rapid expansion and extent of the masses, and the poor prognosis reported for surgical excision, euthanasia was recommended and performed in all cases. Macroscopically, a soft oedematous tissue with a grey to green colour was seen in the regions of missing, malformed or displaced dental precursors or unerupted teeth. Histologically, this tissue consisted of spindle shaped cells surrounded by an abundant homogenous pale stroma, rich in acid mucopolysaccharides. Immunohistochemistry was positive for actin and the mesenchymal marker vimentin. The cheek tooth involvement identified in the described cases resembled what is previously reported in odontogenic myxoma in children and young adults. Even in man, maxillary odontogenic myxoma represents a therapeutic challenge due to the invasiveness of the lesions and radical surgical excision is recommended. Treatment is further complicated by sinonasal involvement. Surgery was not attempted in the cases presented here due to the extent of the lesions and the risk of recurrence. Clinicians should be aware of this rare condition when presented with young horses with sinonasal neoplasia.