Who or What is Egeria

The fountain dedicated to  Egeria is a real place in Rome

 Egeria gave wisdom and prophecy in return for libations of water or milk at her sacred grove, near where the Baths of Caracalla were built in the third century. The name Egeria may mean “of the black poplar.” Egeria was associated by Romans with Diana, and women in childbirth called for her aid, so she appears to have presided over childbirth as well, like the Greek goddess Ilithyia. Egeria was later categorized by the Romans as one of the Camenae, deities who came to be equated with the Greek Muses as Rome fell under the cultural influence of Greece; so Dionysius of Halicarnassus listed Egeria among the Muses. 
A grove sacred to Egeria in connection with Numa stood close by a busy gate of Rome, the Porta Capena. 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. The primeval spring, one of dozens of springs that flow into the river Almone, was made to feed large pools, one of which was known as Lacus Salutaris or “Lake of Health”. Juvenal regretted an earlier phase of architectural elaboration:Nymph of the Spring! More honour’d hadst thou been,If, free from art, an edge of living green,Thy bubbling fount had circumscribed alone,And marble ne’er profaned the native stone.