While at first they may seem not that harmful they exist for one reason: spreading chaos. They do this by infecting other animals, building their numbers up and then assault any human (or duck/etc.) settlement with the intend of utter annihilation.
They rub their dung in their hair, worship gods of disease, and other despicable practices. As a result of their lifestyle, they are immune to poison and disease. Broos tend to roam in small gangs in far-flung regions. A common trick of theirs is to infect money or jewelry with disease to pass it on.
What to look out for
If you get a random event in which a twoheaded calf is born then you need to be careful and give plenty to sacrifice to ward of chaos. Sightings of Broos in neighbouring clanholds also serve as an early warning, send in troops to ensure eradication. If an armed force of Broos is attacking your Tula then get as much help as you can. If Humakti warriors want to hunt in your Tula then let them.