A deer having seven legs is a result of a birth defect or genetic anomaly, often referred to as a polyadactylism. It is a rare occurrence, and the cause is typically a bizarre genetic coding rather than a fusion of two embryos. In the case of a deer with seven legs, extra appendages would likely be visible, potentially attached to the shoulder blades or rear legs. | McGee Equine & Livestock Farrier Service, Rescue, and Rehabilitation