The Hagerman Horse, so named by locals, was discovered in 1928. Scientists call it Equus simplicidens. Five nearly complete skeletons and 100 skulls were retrieved from the site, which is on the hillside across the Snake River from the town of Hagerman. Some paleontologists speculate that a herd of the horses may have been caught in flooding waters and drowned at the site. The Hagerman site remains the largest single discovery of this type of fossil found, and it is the earliest example of Equus, the genus that includes all modern horses, donkeys, and zebras.
It’s not all about the horses. Preserved within the sediments is one of the most diverse deposits of Pliocene animals. Over 100 species of vertebrates, including 18 fish, 4 amphibians, 9 reptiles, 27 birds and 50 mammals have currently been identified, as well as freshwater snails and clams, and plant pollen.
The Idaho Department of Parks and Recreation owned and operated the site for a few years. It was traded for land that became Castle Rocks State Park in 2003. The National Park Service operates the site today as Hagerman Fossil Beds National Monument.
Not every state has a state fossil, but Idaho does. It is, of course, the Hagerman Horse.