Signet rings were frequently used in antiquity as authentication for documents or correspondence, as distinctive handwritten signatures were not yet commonly used for this purpose. The ring would have been pressed into hot wax to seal the document with a personal emblem. Such emblems included motives driven from literature, mythology or scenes of everyday life, as seen on this beautiful example.
For more information about the meanings of animals in Roman art, see our relevant blog post: Animal Symbolism in Roman Art.