Man o’ War was one of the greatest horses in the history of thoroughbred racing. In 1919-1920 the magnificent animal was entered in 21 races and won 20 of them.
The name of the horse that upset Man o’ War was–Upset.
From The Giant Book of More Strange But True Sports Stories by Howard Liss. Illustrations by Joe Mathieu.
https://en.wikipedia.org/wiki/Man_o’_War
According to the Wikipedia article, Man o’ War and Upset raced against each other five times, with Upset winning the one race mostly due to a screw up with the start.