Upon the start of the American Civil War in 1861, offspring of Gaines' Denmark were put into a cavalry troop led by Confederate General John Hunt Morgan.

[2][3] As a stud, Gaines' Denmark first caught the attention of American Saddlebred Horse Association founder and president John B. Castleman in 1857.

Castleman, then a 16-year-old teenager, purchased a 3-year-old, "three-fourths Thoroughbred" gelding named Lightfoot that was sired by Gaines' Denmark out of "a mare by Boston".

With the assistance of Isaac Byrd, an enslaved African American who was owned by Castleman's family, Castleman trained Lightfoot to be a "saddle" show horse, and entered him into a local horse show.

The horse fetched an "unprecedented price", and Castleman became further interested in Gaines' Denmark as a foundational sire for the Saddlebred.