Bulls were a common depiction is Roman art. They were, like other Classical cultures, a symbol of power and fertility. They were also amongst the animals most frequently slaughtered as a sacrificial victim. This practice was associated from the 2nd century AD to the great Mother goddess, to protect the people and the State. Many statuettes of this type have been recovered across the Roman Empire. The subject is thought to be of foreign influence, as either the Apis bull from Ancient Egyptian culture or the sacrificial animal at the centre of the mystic cult of the god Mithras originated in the East. Mithraism was a very popular cult amongst the military in Rome from the 1st up to the 4th century AD.
For more information about the meanings of animals in Roman art, see our relevant blog post: Animal Symbolism in Roman Art.