Passage 3.25.6
Ὅμηρος δὲ--- πρῶτος γὰρ ἐκάλεσεν Ἅιδου κύνα ὅντινα Ἡρακλῆς ἦγεν ---οὔτε ὄνομα ἔθετο οὐδὲν οὔτε συνέπλασεν ἐς τὸ εἶδος ὥσπερ ἐπὶ τῇ Χιμαίρᾳ· οἱ δὲ ὕστερον Κέρβερον ὄνομα ἐποίησαν καὶ κυνὶ τἄλλα εἰκάζοντες κεφαλὰς τρεῖς φασιν ἔχειν αὐτὸν, οὐδέν τι μᾶλλον Ὁμήρου κύνα τὸν ἀνθρώπῳ σύντροφον εἰρηκότος ἢ εἰ δράκοντα ὄντα ἐκάλεσεν Ἅιδου κύνα.
Homer—for he was the first to mention the hound of Hades, which Heracles led forth—does not give it any particular name nor elaborate upon its form as he does with the Chimaera. Later writers, however, assigned to it the name Cerberus, and, likening it otherwise to a dog, declared that it had three heads—though Homer said nothing to indicate more clearly that it was a dog like those that keep men company, than if he had called a serpent the hound of Hades.