Flying Childers - colt by Darley Arabian foaled 1715, widely regarded as the first great thoroughbred racehorse.
The horse didn't race until aged 6 whereupon, lightly raced, he won 6 out of 6 in 3 seasons.
His owner (the 4th Duke of Devonshire) allegedly refused an offer for Flying Childers to pay the horse's weight in gold.