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The Aberdeen Bestiary provides this eminently helpful description of the bonnacon:

In Asia an animal is found which men call bonnacon. It has the head of a bull, and thereafter its whole body is of the size of a bull's with the maned neck of a horse. Its horns are convoluted, curling back on themselves in such a way that if anyone comes up against it, he is not harmed. But the protection which its forehead denies this monster is furnished by its bowels. For when it turns to flee, it discharges fumes from the excrement of its belly over a distance of three acres, the heat of which sets fire to anything it touches. In this way, it drives off its pursuers with its harmful excrement.

This pretty much agrees with what Pliny has to say about the beast:

There is (they say) a wild beast in Pæonia, which is called Bonasus, with a maine like an horse, otherwise resembling a bull: marie, his hornes bend so inward with their tips toward his head, that they serve him in no steed at all for fight, either to offend, or defend himselfe; and therefore, all the helpe that he hath, is in his good footmanship; and otherwhiles in his flight by dunging, which hee will squirt out from behind him three acres in length. This ordure of his is so strong and hot, that it burneth them that follow after him in chase, like fire, if haply they touch it.

Following are some images of the bonnacon, generally showing off its one big fire-pooping trick. See also The Medieval Bestiary: Bonnacon.