Zoomorphic brooches were a common theme and make up a substantial amount of the repertoire available. Aquatic animals were popular, although frogs are more scarce than other motifs. Frogs appear in both Roman mythology and Celtic. To the former, they were emblems of fertility and harmony between two lovers. Thus they were dedicated to Venus and imbued with a sense of licentiousness. To the Celts, frogs were associated with rivers, wells and water, making them a sacred emblem. They were thought to bring good luck and harmony and were often worn for their apotropaic values.
Brooches such as this were most arguably continental in origin, rather than made in Britain by local craftsmen.
For more information on ancient Roman animal symbolism, please see our blog post: Animal Symbolism in Roman Art