Bronze objects were ubiquitous in the Roman world, being used by the Roman army, for instance, for both practical and decorative purposes. The phalera is the sculpted disk (usually made of bronze) on the breastplate, which was worn by Roman soldiers as a reward for outstanding contribution, or as a signifier of military rank.
During the Roman Empire, horses were extremely important for battle, as well as for aspects of everyday life, such as transportation, hunting, farming, and chariot racing. The Romans associated the horse with the spoils of war, connecting it symbolically with power, victory, honour, domination, and virility. In Graeco-Roman mythology and culture, the horse was said to have been created by Poseidon (Neptune) and devoted to Hades (Pluto) and Ares (Mars). The Romans also believed the horse to be a symbol of the continuity of life, and would sacrifice a horse to the god Mars every October, keeping its tail through the winter as a sign of fertility and rebirth.
For more information about the meanings of animals in Roman art, see our relevant blog post: Animal Symbolism in Roman Art.