Sifrhippus (image) University of Nebraska-Lincoln Caption This is an artist's reconstruction of Sifrhippus sandrae (right) touching noses with a modern Morgan horse (left) that stands about 5 feet high at the shoulders and weighs about 1,000 pounds. Sifrhippus was the size of a small house cat (about 8.5 pounds) at the beginning of the Eocene (approximately 55.8 million years ago) and is the earliest known horse. Credit Danielle Byerley, Florida Museum of Natural History. Usage Restrictions None Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.