Sources : Cacus

Virgil [circa 1000 BCE] (Aeneid, 8.184): There was a cave here, receding to vast depths, / untouched by the sun's rays, inhabited by the fell shape / of Cacus, the half-human, and the ground was always warm / with fresh blood, and the heads of men, insolently / nailed to the doors, hung there pallid with sad decay. / Vulcan was father to this monster: and, as he moved / his massive bulk, he belched out his dark fires. / Now at last time brought what we wished, the presence / and assistance of a god. Hercules, the greatest of avengers, / appeared, proud of the killing and the spoils of three-fold / Geryon, driving his great bulls along as victor, / and his cattle occupied the valley and the river. / And Cacus, his mind mad with frenzy, lest any / wickedness or cunning be left un-dared or un-tried / drove off four bulls of outstanding quality, and as many / heifers of exceptional beauty, from their stalls. / and, so there might be no forward-pointing spoor, the thief / dragged them into his cave by the tail, and, reversing / the signs of their tracks, hid them in the stony dark: / no one seeking them would find a trail to the cave. - [ Kline translation]

Thomas of Cantimpré [circa 1200-1272 CE] (Liber de natura rerum, Quadrupeds 4.20): The cacus is a monster in Arcadia. This entire animal is bristly just like a boar. And just like the philosopher Andelmus writes, he gushes forth flames from his chest, and it is a puffing and fiery breathing. Also he does this particularly when he is stirred up by anger. But one is able to question, in what way might he be able to vomit out flames or fiery breath from his mouth and in what way those flames are effective for burning away the things they touch, without the body of the animal not to be consumed by the flames. We are able to respond that the breath is is spread widely in the innermost part of an animal and through many diverse organs, and is warm in every conceivable way, but does not explode into flames; but the belching of heat continues and multiplies into an object, as for example, just like when we look through the a beryl stone, the light of the sun's rays is increased, and at its highest peak the united rays cause flames to take up power in the underlying matter and become a violent fire. This is similar to those animals, of which the divine scripture in the book of Wisdom testifies, that breathe fire from their mouths. The monster Cacus live in caves above the River Tiber. This animal suddenly attacks herds of cows and bulls, and one animal from the herd is not sufficient for him, but he seizes three or four bulls by their tails and drags them into his cave with his strength of valor, secretly, walking backwards lest he may be able to be tracked down with ease. Moreover this animal is dangerous not only to animals, but also he lies in ambush even for that strongest man, while still he greatly fears those men. The Cacus represents certain kinds of angry people, who indeed seem first to grow angry little by little within, but when they are not able to restrain their own selves from their increasing anger, just like flames they belch forth insults and disgraces in this way, so that they not only inflame themselves, but also many people having been conquered by their own anger approaching hatred. And thus so that he misses no opportunity for his own wickedness, he draws many people toward the gates of Hell with his backwards step, because no hatred can be noble or just. Therefore the angry one is dangerous not only to brutish humans, in that he may provoke them to irascibility, but also sometimes with rational and religious men, because indeed one must be very excellent whom anger does not overthrow at any time. - [Badke translation/paraphrase]