Lewis & Short

sŏnĭ-pēs (sŏnŭpēs), pĕdis, adj. [sonus], with sounding feet, noisy-footed (poet.): turba (in Isidis sacris), Grat. Cyn. 43.
As subst., noisy-foot, i. e. a horse, a prancer, steed: fremit aequore toto Insultans sonipes et pressis pugnat habenis, Verg. A. 11, 600; Cat. 63, 41; Sil. 1, 222; Val. Fl. 3, 334; Stat. Th. 5, 3 al. (in Cic., sonipedes is cited as an example of a fourth pæon, de Or. 3, 47, 183).