Have used Ketamine on an injured horse that was a very bad kicker and he could not be tended to unless he was stoned out of its head.
Often it's mixed with a bit of Valium and Bob's your Uncle, so to speak. Maybe the old horse had to be sedated for some reason and they raced him too early? Ketamine shows up even in the most mintute of minute amounts as the test for it is basically an Equine adapted Human Drugs Of Abuse platform & as such it is similarly extremely sensitive.