Bonnacon

The bonasus/bonnacon 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.

-- Pliny the Elder

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

-- Aberdeen Bestiary

 In the Ravnica campaign setting, the Bonnacon is often used in Rakdos shows, with it's anal flames allowing for both their characteristic pyrotechnics, and the humor of fart jokes.

Optionally, a DM may allow the feces of the Bonnacon to be used in crafting a flametongue.