Phosphorous was by Eclipse out of Herod, grand dam of Matchem. He was purchased as a two-year old for 200 guineas and showed early promise in training by beating three of the best trial horses. He suffered an injury and never raced. He was purchased by Mr. Taplin, who hunted him with the King's Staghounds during the 1797-98 season.
According to Aubrey Noakes, this picture was painted between 1797 and 1800 (p. 31).