What is a potential cause for dark proliferation around the anus in an Arabian horse?

The presence of dark proliferation around the anus in an Arabian horse is most commonly associated with melanoma. Melanomas are benign tumors that arise from melanocytes, the pigment-producing cells in the skin. Arabian horses are particularly predisposed to developing melanocytic tumors, especially as they age. These tumors can appear as dark, pigmented bumps or masses and can develop around various areas, including the anus.

The diagnosis generally involves a veterinary examination and may be confirmed with a biopsy if necessary. Understanding the predisposition of Arabian horses to melanoma helps in recognizing and monitoring any skin lesions that may indicate potential malignancy or complications associated with these tumors.

Warts could be another consideration, but they typically present differently and are caused by viral infections, which are not specifically associated with the dark pigmentation seen in melanomas. Hemorrhoids are typically not a concern in horses, and a fungal infection would usually show signs of infection rather than a dark proliferation. Therefore, melanoma is the most fitting answer regarding dark proliferation in this context.