[5] Egeria as a nymph or minor goddess of the Roman religious system is of unclear origin; she is consistently, though not in a very clear way, associated with another figure of the Diana type; their cult is known[6] to have been celebrated at sacred groves, such as the site of Nemi at Aricia, and another one close to Rome (see section below); both goddesses are also associated with water bearing wondrous, religious or medical properties (the source in that grove at Rome was dedicated to the exclusive use of the Vestals[7]); their cult was associated with other, male figures of even more obscure meaning, such as one named Virbius,[8] or a Manius Egerius, presumably a youthful male, that anyway in later years was identified with figures like Atys or Hippolyte, because of the Diana reference (see Frazer).

This quality has been made especially popular through the tale of her relationship with Numa Pompilius (the second legendary king of Rome, who succeeded its founder Romulus).

According to mythology, she counseled and guided the King Numa Pompilius (Latin numen designates "the expressed will of a deity"[9]) in the establishment of the original framework of laws and rituals of Rome.

According to Ovid's Metamorphoses, with Numa's death, Egeria melted into tears of sorrow, thus becoming a spring (...donec pietate dolentis / mota soror Phoebi gelidum de corpore fontem / fecit... [18]), traditionally identified with the one nearby Porta Capena in Rome.

[19] The ninfeo, a favored picnic spot for nineteenth-century Romans, can still be visited in the archaeological Park of the Caffarella, between the Appian Way and the even more ancient Via Latina,[20] nearby the Baths of Caracalla (a later construction).

In the second century, when Herodes Atticus recast an inherited villa nearby as a great landscaped estate, the natural grotto was formalized as an arched interior with an apsidal end where a statue of Egeria once stood in a niche; the surfaces were enriched with revetments of green and white marble facings and green porphyry flooring and friezes of mosaic.