Augustine, City of God 18.17:
In order to add to this, Varro relates many other things no less incredible about that sorceress Circe, who turned even the companions of Ulysses into beasts, and about the Arcadians who, drawn by lot, swam across a certain pool and were there turned into wolves who lived with similar beasts through the deserts of that region. If, however, they did not eat human flesh, and swam back across the same pond after a period of nine years, they were turned back into humans. Finally, he even described by name a certain Demaenetus taking a taste of the sacrifice which the Arcadians were accustomed to make to their god Lycaeus by sacrificing a boy. Demaenetus was turned into a wold and on the tenth year restored to his proper form, in which he practiced boxing and won in the Olympic contest. This same historian thinks that the names Pan Lycaeus and Jupiter Lycaeus are applied in Arcadia for no other reason except this mutation of people into wolves, which they thought could not happen except through divine force. For a wolf is called a lykos in Greek, from which the name Lycaeus seems to be derived. Varro adds that the Roman Luperci arose as if from the seeds of those mystery rites.
Hoc Varro ut astruat, commemorat alia non minus incredibilia de illa maga famosissima Circe, quae socios quoque Vlixis mutauit in bestias, et de Arcadibus, qui sorte ducti tranabant quoddam stagnum atque ibi conuertebantur in lupos et cum similibus feris per illius regionis deserta uiuebant. Si autem carne non uescerentur humana, rursus post nouem annos eodem renatato stagno reformabantur in homines. Denique etiam nominatim expressit quendam Demaenetum gustasse de sacrificio, quod Arcades immolato puero deo suo Lycaeo facere solerent, et in lupum fuisse mutatum et anno decimo in figuram propriam restitutum pugilatum sese exercuisse et Olympiaco uicisse certamine. Nec idem propter aliud arbitratur historicus in Arcadia tale nomen adfictum Pani Lycaeo et Ioui Lycaeo nisi propter hanc in lupos hominum mutationem, quod eam nisi ui diuina fieri non putarent. Lupus enim Graece lukos dicitur, unde Lycaei nomen apparet inflexum. Romanos etiam Lupercos ex illorum mysteriorum ueluti semine dicit exortos.