From Plato’s Republic, Book 8 (565d)
“What is the beginning of the change from guardian to tyrant? Isn’t clear when the guardian begins to do that very thing which myth says happened at the shrine of Lykaion Zeus in Arcadia?
Which is? He said.
That once someone tastes a bit of human innards mixed up with the other sacrifices he becomes a wolf by necessity? Haven’t you heard this tale?
Is it not something the same with a protector of the people? Once he controls a mob that obeys him, he cannot restrain himself from tribal blood, but he prosecutes unjustly, the sorts of things men love to do, and brings a man into court for murder, eliminating the life of a man—and with tongue and unholy mouth that have tasted the murder of his kind, he exiles, kills, and promises the cutting of debts and the redistribution of land. Is it not by necessity that such a man is fated either to be killed by his enemies or to become a tyrant, to turn into a wolf from a man?”
Τίς ἀρχὴ οὖν μεταβολῆς ἐκ προστάτου ἐπὶ τύραννον; ἢ δῆλον ὅτι ἐπειδὰν ταὐτὸν ἄρξηται δρᾶν ὁ προστάτης τῷ ἐν τῷ μύθῳ ὃς περὶ τὸ ἐν ᾿Αρκαδίᾳ τὸ τοῦ Διὸς τοῦ Λυκαίου ἱερὸν λέγεται;
῾Ως ἄρα ὁ γευσάμενος τοῦ ἀνθρωπίνου σπλάγχνου, ἐν ἄλλοις ἄλλων ἱερείων ἑνὸς ἐγκατατετμημένου, ἀνάγκη δὴ τούτῳ λύκῳ γενέσθαι. ἢ οὐκ ἀκήκοας τὸν λόγον;
῏Αρ’ οὖν οὕτω καὶ ὃς ἂν δήμου προεστώς, λαβὼν σφόδρα πειθόμενον ὄχλον, μὴ ἀπόσχηται ἐμφυλίου αἵματος, ἀλλ’ ἀδίκως ἐπαιτιώμενος, οἷα δὴ φιλοῦσιν, εἰς δικαστήρια ἄγων μιαιφονῇ, βίον ἀνδρὸς ἀφανίζων, γλώττῃ τε καὶ στόματι ἀνοσίῳ γευόμενος φόνου συγγενοῦς, καὶ ἀνδρηλατῇ καὶ ἀποκτεινύῃ καὶ ὑποσημαίνῃ χρεῶν τε ἀποκοπὰς καὶ γῆς ἀναδασμόν, ἆρα τῷ τοιούτῳ ἀνάγκη δὴ τὸ μετὰ τοῦτο καὶ εἵμαρται ἢ ἀπολωλέναι ὑπὸ τῶν ἐχθρῶν ἢ τυραννεῖν καὶ λύκῳ ἐξ ἀνθρώπου γενέσθαι;
In ancient Greek myth, Lykaon (Lycaon, related to lúkos, “wolf”) was a king of Arcadia. According to Pausanias (8.31-5) , Lykaon sacrificed a newborn child to Zeus. In other sources he offers the infant mixed up with other food to test Zeus’ divinity (although some attribute the deed to his sons, see Apollodorus, 3.8.1). Zeus killed the sons with lightning; Lykaon was transformed into a wolf.
There are other sources (Pausanias, Pliny and Petronius) who mention other lycanthropies, but this one seems like an appropriate starting point.