City of God
Augustine, What Varro Says of the Incredible Transformations of Men, 18.17 from the Christian Classics Ethereal Library.
In support of this story, Varro relates others no less incredible about that most famous sorceress Circe, who changed the companions of Ulysses into beasts, and about the Arcadians, who, by lot, swam across a certain pool, and were turned into wolves there, and lived in the deserts of that region with wild beasts like themselves. But if they never fed on human flesh for nine years, they were restored to the human form on swimming back again through the same pool. Finally, he expressly names one Demaenetus, who, on tasting a boy offered up in sacrifice by the Arcadians to their god Lykaios according to their custom, was changed into a wolf, and, being restored to his proper form in the tenth year, trained himself as a pugilist, and was victorious at the Olympic games. And the same historian thinks that the epithet Lykaios was applied in Arcadia to Pan and Jupiter for no other reason than this metamorphosis of men into wolves, because it was thought it could not be wrought except by a divine power. For a wolf is called in Greek lykòs, from which the name Lykaios appears to be formed. He says also that the Roman Luperci were as it were sprung of the seed of these mysteries.
From The Latin Library.
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 *lu/kos dicitur, unde Lycaei nomen apparet inflexum. Romanos etiam Lupercos ex illorum mysteriorum ueluti semine dicit exortos.