From Wiktionary, the free dictionary
From super- + horse.
superhorse (plural superhorses)
- An extremely famous or talented horse, especially a racehorse
2008 May 4, William C. Rhoden, “Race Illustrates Brutal Side of Sport”, in New York Times[1]:Matz was the trainer of Barbaro, the superhorse who won here in 2006 and took that fatal misstep two weeks later at the Preakness.