Even pet dogs can become quite vicious and feral when allowed to roam free. My brother lived on an acreage outside of a major Canadian city and owned two fairly large dogs. Unfortunately, although he tried to keep them penned up, they got loose on occasion. After the third or fourth occasion, one of his neighbours hinted strongly that two dogs that "looked a lot like" my brother's pets had been part of a gang of dogs that had killed some of his livestock.

My brother took the hint and had both dogs put down.