Believed to be specific to the southwest. Average size was estimated around 1100lb. About the size of a mustang Bibliography: Klide, A. M. (1989). “Overriding vertebral spinous processes in the extinct horse, Equus occidentalis“. American Journal of Veterinary Research. 50 (4): 592–593. PMID2712429. “Equus occidentalis”. http://www.utep.edu.