WinStar Farm stallion Improbable (City Zip), the champion older male of 2020 and a 'TDN Rising Star', was euthanized late last night at Rood & Riddle Equine Hospital in Lexington after suffering from ataxia behind, a condition which causes poor voluntary muscle control, the farm said in a release early Sunday. "He was sent to Rood and Riddle yesterday morning to determine the exact cause of the ataxia," said Elliott Walden, president, CEO, and racing manager of WinStar Farm. "He was responding well to treatment, but went to lie down...