Such glass pendants would have been worn by Romans as good fortune charms to bring good luck and ward off the evil. Iconographies were extremely differentiated, including apotropaic symbols, images of deities and animals. The Romans associated frogs with fertility and harmony, as well as connecting them with Venus and licentiousness. Frogs were also important characters in the fables of Aesop and Phaedrus.
For more information about the meanings of animals in Roman art, see our relevant blog post: Animal Symbolism in Roman Art.