Purchased by the partnership of Jack P. Chinn and George W. Morgan, and thereafter stabled in Mercer County, Kentucky, Leonatus was trained by African American horseman Raleigh Colston Sr. as well as by John McGinty.

Although there were no official awards given until 1936, Leonatus was retrospectively chosen the American Champion Three-Year-Old Male Horse of 1883.

Carrying 105 pounds and the 2-1 favorite, Leonatus was piloted by Billy Donohue, the jockey who'd ridden Sligo in the 1881 Derby, coming in fourth.

Running in mud on a clear, cold day, Leonatus was up against the colt Drake Carter, trained by Green B. Morris, who hoped to take his second consecutive Derby (he'd won with Apollo in 1882).

Leonatus lies in an unmarked grave on a bend of the Elkhorn Creek that runs through Runnymeade Farm.