Cutaneous apocrine adenocarcinoma is a very rare tumor in horses. Only two cases have been previously reported. In this case, there were several unusual findings which emerged during the course of diagnosis and treatment and which have not been previously reported in association with this type of tumor. These findings included possible humoral hypercalcemia of malignancy and concurrent cutaneous lesions which may have been associated with the tumors. The patient was treated with a new form of combination therapy: surgical excision with intraoperative local injection of cisplatin. This report describes the patient's case history, diagnostic workup, treatment, and case outcome.