The 72million-year-old rhino-sized plant-eater Coahuilaceratops magnacuerna was 22ft long, 6ft to 7ft tall and weighed in at five tons.
It was an ancestor of the famous three-horned Triceratops. The Coahuilaceratops had two enormous horns that jutted out from above its eyes.
The fossil bones of the adult and one of its young were found in Coahuila, southern Mexico.
Dr Mark Loewen, from the Utah Museum of Natural History in Salt Lake City, US, said: “Finding this horned dinosaur so far south in Mexico offers us a different picture of what the ancestors of Triceratops were like.”
The discovery site is desert but 72million years ago was an estuary with lush vegetation.