Equus simplicidens, sometimes known as the Hagerman horse or the American Zebra is an extinct species of equine native to North America during the Pliocene and Early Pleistocene.
It was first discovered in 1928 near Hagerman, Idaho, and is known for its large size and robust build. It is the state fossil of Idaho.
The Hagerman Horse stood about 43–57 in (1.2–1.4 m) high at the shoulder and weighed up to 2,000 pounds.
The Hagerman fossils represent some of the oldest and most primitive (4 to 3 million years ago) widely accepted remains of the genus Equus.
The Hagerman Horse was very similar to the Zebra of East Africa today. It had one toe or hoof, a thick body, and neck, a head similar to a donkey, and may have had a short stiff mane like the modern zebra.
A cattle rancher named Elmer Cook discovered some fossil bones in 1928 on this land in Hagerman, Idaho.
Identified as bones belonging to an extinct horse, the area where the fossils were discovered, called the Hagerman Horse Quarry, was excavated extensively.
Excavation of the fossils continued into the early 1930s and ultimately five nearly complete skeletons, more than 100 skulls, and forty-eight lower jaws as well as numerous isolated bones were found.
Some paleontologists believed that such a large amount of fossils found in one location was because of the quarry area being a watering hole at one point where injured, old, and ill animals, drawn to water, died.
Other paleontologists think that an entire herd of these animals drowned attempting to ford a flooded river and were swept away in the current and ended up buried in the soft sand at the bottom.
A 2019 phylogenetic analysis found it to be more closely related to living Equus than to Hippidion or Dinohippus, but outside the group containing all living equines.
The Hagerman Horse likely became extinct due to a combination of factors such as climate change, habitat loss and competition with other species.
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