Which of the following is NOT a symptom of Cushing's Syndrome in horses?

Weight gain is not a typical symptom of Cushing's Syndrome in horses, making this the correct choice. In fact, Cushing's Syndrome is more commonly associated with weight loss and a potbellied appearance due to muscle wasting. Horses affected by this condition often show symptoms such as excessive urination, laminitis, and hirsutism (which refers to an abnormal growth of hair). It’s important to recognize that while some horses may experience weight fluctuations, the hallmark of Cushing's tends to be more toward losing weight rather than gaining it. This understanding is crucial for proper diagnosis and management of the condition in affected horses.