Phaëthusa was the daughter of Helios who with her sister Lampetia guarded her father's cattle in Sicily. The name was also that of the daughter of Danaüs, who became the mother of Myrtilus by Hermes.

The meaning of the name is 'bright being', from phaetho, 'to shine' and ousia, 'being'. For Helios' daughter this is clearly an appropriate name.