Synthetoceras tricornatus is a large, extinct protoceratid, endemic to North America ( Nebraska ) during the Late Miocene, 10.3—5.3 Ma, existing for approximately. Fossils have been recovered from Nebraska and Texas.

Description

With a length of 2 m and a mass of 150 –, Synthetoceras was the largest member of its family. It was also the last, and had what is considered to be the protoceratids' strangest set of horns. The two horns above its eyes looked fairly normal and similar to those of many modern horned mammals, but on its snout it had a bizarre, long horn with a forked tip that gave it a Y-shape. Only males had this strange horn, and they probably used it in territorial fights.