The Bonnacon (Bonacon, Bonasus, or Vilde Kow)
Only the Anglo-Saxon word can do this creature semantic justice in English. It sprays fiery shit from its nethers. The medieval bestiaries are clear on this matter:
Even to us today, the humor and disgust are evident. I presume that the bonnacon is the inspiration for the stench kine (singular, kow), the cattle that roam D&D's nine hells. But the medieval Europeans inherited this foul bovine from their beloved Pliny, who passes this on: "There are reports of a wild animal in Paionia called the bonasus, which has the mane of a horse, but in all other respects resembles a bull; its horns are curved back in such a manner as to be of no use for fighting, and it is said that because of this it saves itself by running away, meanwhile emitting a trail of dung that sometimes covers a distance of as much as three furlongs [604 m], contact with which scorches pursuers like a sort of fire" (Naturalis Historia, Book 8, 6) That's more than 1200 feet away. Four playing fields between you and a bonnacon and you'd still be in danger, depending on whether you're thinking American or Association Football. Take a look at The Medieval Bestiary and The Aberdeen Bestiary Project. The text of the latter seems to agree with Pliny that it is contact that causes combustion, but with the "fumes" rather than the dung itself.
It is clear that a creature this awesomely awful deserves an honorary place in mythopoesis. You can decide if they were they fall on the continuum from mere rumor to straightforward magical creature within your imaginary world. I can't help but relate them to another mythic bovine, the cattle of the Sun (appearing most famously in Homer's Odyssey xii.127–137), although one could also relate them to the medieval (bovine) gorgon, the biblical golden calf, or the Hindu taboo regarding cows. I have added a Rumor Table to the Dungeon of Game-Mastery. GMs can decide which parts of the rumors are true and which are false for themselves, just as I will for Ygg. (While I've tailored the rumors to my world of Ygg, those few details can be changed as needed for other worlds by GMs who want to adapt them.) Later this week I will add stats for a S&W version of the bonnacon that I have been working on, perhaps with further Ygg-specific text, in the same section.
Here's wishing you a merry Mythopoeic Monday!
NB. I have tried to name each file for its origin. There are a couple that I cannot trace, but if anyone informs me about these, I will correct the names.
EDIT: Here is a link to a creature (the charnel cow) for Pathfinder RPG that was put together by two pals of mine, Mike W. and Sarah C., inspired by the stench kow mentioned above. Thanks for your faithful support of M.R. and of consumers of fine (er, disgusting) monsters everywhere!