It’s true; I have never heard of any of these fabulous medieval creatures before. The descriptions all compare parts of the beast to those of other known animals, which was necessary but not at all accurate. Then the animals are dressed up with strange powers from someone’s nightmares. There’s one that can move its horns around independently, another with wings even though it’s a fish, and then there’s the Bonnacon, a bull of sorts with a built-in flamethrower.
According to the Aberdeen Bestiary, "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."