Melanocytic neoplasms in a black rhinoceros (Diceros bicornis) and an Indian rhinoceros (Rhinoceros unicornis).

Melanocytic neoplasms were diagnosed in a captive black rhinoceros (Diceros bicornis) and a captive Indian rhinoceros (Rhinoceros unicornis) from different facilities. The first case was a 10-yr-old, captive-born male black rhinoceros that presented with a small firm cutaneous mass on the dorsal midline. Aspirate cytology results were suggestive of a melanocytic neoplasm, and histologic examination of the excised mass confirmed a well-differentiated neoplasm with much pigment production, minimal anaplasia, and no mitotic figures. Several months after mass removal, a similar mass with identical histologic features was excised from the right medial thigh. The second case was a 28-yr-old, wild-born female Indian rhinoceros that presented with a draining wound at the coronary band of a rear digit. Histologic examination of a biopsy from this lesion revealed a melanocytic neoplasm with moderate cellular anaplasia, frequent mitoses, and scant pigment production. At necropsy, the tumor was found to ablate P3 and most deep tissues of the toe. No evidence of vascular invasion or metastasis was found. These two cases represent the only melanocytic neoplasms in Rhinoceridae reported in detail in the literature.