The cavalry was a minor part of the Roman army, whose main strength lay in the heavy infantry—the legions and their auxiliary cohorts. Some horsemen were attached to each legion, however, and there were special units, called alae, recruited from among barbarian nations that excelled in riding. The emperor and senior officers, too, had mounted escorts.
Richter, Gisela M. A. 1915. Greek, Etruscan and Roman Bronzes. no. 1605, pp. 428-29, New York: Gilliss Press.
Littauer, M.A., Joost H. Crouwel, and Vassos Karageorghis. 2002. "Notes on Prometopidia." Selected Writings on Chariots, Other Early Vehicles, Riding and Harness, Peter Raulwing, ed. p. 493 n. 4, pl. 208, Leiden: Brill.