Etymologiae, Book I, chapter 40: De fabula
Chapter XL. On fable The poets named fables from speaking (fando), because they are not things done but things merely feigned in speech. They were introduced so that, by the conversation of feigned dumb animals with one another, a certain image of human life might be recognized. Alcmaeon of Croton is said to have invented them first, and they are called Aesopic because Aesop was eminent in this matter among the Phrygians. Fables are either Aesopic or Libystic. Aesopic ones are when dumb animals are feigned to converse with one another, or things which have no soul, such as cities, trees, mountains, rocks, rivers. Libystic ones, when an exchange of speech is feigned between men and beasts or beasts and men. Some fables the poets feigned for the sake of delighting, some they interpreted with respect to the nature of things, some with respect to the manners of men. Feigned for the sake of delighting, such as those which are commonly told, or such as Plautus and Terence composed. They feign fables with respect to the nature of things, as “lame Vulcan,” because fire by nature is never straight, like that three-formed beast: “Lion in front, dragon behind, in the middle the Chimaera herself”—that is, a she-goat—wishing by it to distinguish the ages of men; of which the first, adolescence, is fierce and bristling, like the lion; the middle time of life is the brightest, like the she-goat, because she sees most keenly; then old age comes with bent chances, the dragon. So also the fable of the Hippocentaur, that is, a man mixed with a horse, was devised to express the swiftness of human life, since the horse is known to be the swiftest. With respect to manners, as in Horace the mouse speaks to the mouse and the weasel to the little fox, so that through a feigned narration a true signification may be brought to what is in hand. Hence Aesop’s fables are of this kind, referred to the end of morals; or as in the book of Judges the trees seek a king for themselves and speak to the olive and to the fig and to the vine and to the bramble; all of which is surely feigned for morals, so that one comes to the matter intended by a narration feigned indeed but of true signification. So also the orator Demosthenes used a fable against Philip, who, when he demanded from the Athenians that ten orators be given to him, and that he would withdraw, feigned this fable to dissuade them, saying that the wolves once persuaded the shepherds, whose diligence they wished to deceive, to come into friendship, on the condition however that the dogs, who were the cause of the quarrels, should by right be handed over to them: the shepherds agreed and, in hope of security, gave up the dogs, whom they had as the most watchful guardians of their sheep. Then the wolves, all fear removed, tore everything that was in those flocks, not only for satiety but also for pleasure. Philip likewise demanded the leaders of the people, that he might the more easily crush the city stripped of its guardians.