Two teams of researchers, including a scientist from Case Western Reserve University in Cleveland, say the species, about the size of a small zebra, had three-toed hooves and grazed the grasslands and shrubby woods in Ethiopia's Afar Region.
The new species has been named Eurygnathohippus woldegabriel for geologist Giday WoldeGabriel, who earned his doctorate at Case Western Reserve in 1987 and is noted for helping unravel the geological complexities of the fossil deposits in the Ethiopian Rift system.
The fossil not only fills a gap in the evolutionary history of horses but is also important for documenting how old a fossil locality is and in reconstructing habitats of human forebears of the time, said study co-author Scott Simpson, a professor of anatomy at Case Western Reserve's School of Medicine.
"This horse is one piece of a very complex puzzle that has many, many pieces," he said, noting the fossil horse was among the diverse array of animals that lived in the same areas as the ancient human ancestor Ardipithecus ramidus.
Fossilized bones of E. woldegabriel indicate this ancient species was an adept runner, the researchers said, an evolutionary trait helping it to flee lions, sabre-tooth cats and hunting hyenas that would run down their prey.