Spend any time reading Medieval Bestiaries you will run into all sorts of fantastic animals such as dragons, unicorns, griffins, and the bonnacon.
Ah. The majestic bonnacon. It is a large bull-like creature with inward-turned horns, the mane of a horse, and it attacks by shooting flaming caustic dung at you.
Wait. What?
Yes, the bonnacon (also called bonasus or bonacho) is a great mythic beast that has appeared in numerous bestiaries. Its horns are useless for defense, it instead will shoot caustic feces out of its anus while it is running away.
It was first described by none other than Pliny the Elder in his Naturalis Historia. This history is a great source of monsters.
And there are some GREAT pictures of this awful beast.
Bonnacon
Large Beast (Magical)
Frequency: Rare
Number Appearing: 1d4 (1d6)
Alignment: Neutral [Unaligned]
Movement: 150' (50') [5"]
Armor Class: 6 [13]
Hit Dice: 2d8+2* (11 hp)
Large 2d10+2* (13 hp)
Attacks: 1 head butt or special
Damage: 1d6
Special: Caustic dung
Size: Large
Save: Monster 2
Morale: 6 (8)
Treasure Hoard Class: None
XP: 35 (OSE) 47 (LL)
Str: 14 (+1) Dex: 12 (0) Con: 15 (+1) Int: 4 (-2) Wis: 7 (+1) Cha: 5 (-2)
The bonnacon is a large bull-like creature. It is typically red or brown in color with a long horse-like mane running from its head, down the back of its neck. They have two large bull-like horns, but they are turned inside and thus provide no effective means of protecting the creature. The bonnacon is also spectacularly stupid, even judging it compared to other heard animals.
The bonnacon can attack with a head butt but would rather run away. It will use its only special attack at this point. When retreating the bonnacon will eject burning, caustic dung from its anus. This dung will stick to clothes, skin, and just about everything. When hit the victim must save vs poison or take 4d6 hit points of damage; save for half. The dung will continue to burn any skin it touches causing 1d6 hp of damage per round. A previous save means that no skin was touched. The only way to remove these caustic feces is to wash them off with at least a quart of water. A running stream or a decanter of endless water is also good. Thankfully a bonnacon can only use this attack once per day.
The meat of the bonnacon is vile and rank. Goblins, who can eat anything, will not eat the meat of this animal. It does however eat a lot on its own. So a small herd (1d4) can destroy up to 40 lbs of grain or plants per day each.