Straight from the horse's mouth

The leading theory traces this to British horse racing. Tips about a horse's chances came from all sorts of people, trainers, jockeys, stable boys, but the joke was that the only source more reliable than the people closest to the horse would be the horse itself. A competing theory points instead to the old practice of judging a horse's age and health by directly examining its teeth, since a seller's word about a horse could be flattering, but the animal's own mouth couldn't lie. The earliest identified citation is from a horse-racing publication, Bell's Life in London and Sporting Chronicle, in 1861, which lends real weight to the racing-tip theory, though it doesn't fully rule out the other.