In his Histories, Polybius compares the descent of the rule of King Philip v of Macedon to tyranny to Plato’s description from the Republic of a man transforming into a werewolf. Such imagery is unique in classical Greek historiography, and exemplifies Polybius’ reliance on the idea of men acting like animals to describe when individuals or groups lose self-restraint, an idea found in Plato’s Republic. Plato uses θηριώδης to describe the ‘base desires’ of the soul that must be constrained by reason otherwise the individual will resort to crime or political revolution to satiate them. Polybius employs ἀποθηριόω in situations when individuals or groups lose part or all of their self-control, which results in self-destruction. The parallels in language and ideas indicate that Polybius’ idea of humans acting like animals derives from Plato. Recognizing this intellectual origin provides readers with a better understanding of the universal lessons of the Histories.