Sources : Bonnacon
Aristotle [ca. 350 BCE] (De animalibus Book 9, chapter 32.1-3): The bonassus is found in Pæonia, in Mount Messapius, which forms the boundary between Pæonia and Mædia. The Pæonians call it monapus. It is as large as a bull, and more heavily built; for it is not a long animal... In form it resembles a bull, but it has a mane as far as the point of the shoulder like the horse, but its hair is softer than that of the horse, and shorter. ... Their voice is like that of the ox. Their horns are crooked and bent together, of no use for defence, a span long or a little more, so thick that each of them would hold half a measure or a little more. ... When wounded it retreats, and stays when it can proceed no farther. It defends itself by kicking and ejecting its dung, which it can do to the distance of four fathoms from itself. It uses this means of defence easily and frequently. Its dung is so caustic as to burn the hair from dogs. The dung is only caustic when the creature is disturbed and alarmed. It is not so when undisturbed. - [Cresswell translation, 1887]
Pliny the Elder [1st century CE] (Natural History, Book 8, 16): The bonasus is found in Paeonia. It has the mane of a horse but otherwise resembles a bull. It has horns that curve back so they are useless for fighting; when attacked, it runs away, while releasing a trail of dung that can cover three furlongs. Contact with the dung burns pursuers as though they had touched fire.
Aelianus [170-230 CE] (On the Characteristics of Animals, Book 7, chapter ): There is an animal in Paeonia called Monops, and it is the size of a shaggy bull. Now when this creature is pursued, in its agitation it excretes a fiery and acrid dung, so I am told; and should this happen to fall on any of the hunters, it kills him. - [Scholfield translation]
Gaius Julius Solinus [3rd century CE] (De mirabilibus mundi / Polyhistor, Chapter 40.10): In these parts [Phrygia] an animal called the bonacus is produced. Its head and the body which follows is like that of an ox, except that it has a mane like a horse’s. Its horns run back on themselves with such multitudinous winding that if someone bumps against them, he is not wounded.  The protection the forehead denies to this monster is provided by its belly. When it turns to flee, it discharges dung with a quick evacuation of its stomach, for a length of three acres. The heat of the dung burns whatever it touches. Thus it wards off pursuers with its noxious secretion. - [Arwen Apps translation, 2011]