Polydactyly is a genetic disorder characterized by partial or complete duplication of a digit. Studies in humans, cattle, dogs and cats indicate that an autosomal dominant gene of incomplete penetrance is responsible for this change. Polydactyly is rare in horses and its cause remains unclear. However, when it occurs it is related to congenital malformation of the phalanx. This paper describes, through the observation of physical, radiographic and ultrasound finds, the occurrence of unilateral polydactyly, with unusual features in a nine year old horse. Since birth, the animal, which had a history of severe lameness, had complete division of the digit of the right forelimb, with symmetry between the two distal portions. The radiographic examination identified the presence of two separated digits with similar size, which is an unusual manifestation of polydactyly in the equine species.
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