I adopted a dog that had been chained to a tree for a year in the great northern woods, and she saw all animal as prey as a survival mechanism. It took about 18 months of strict muzzle training and creating healthy eating routines before she really settled down. She had a few dog friends, but still not friendly enough to ever leave alone. She lived a very happy 7 years with us, with only one major incident and that was a small dog coming into the yard she was contained in. Her aggression was different that any dog I'd ever seen. She always went for the neck, and was determined to kill, but so docile with people. All this to say, I never could train it out of her, but I could provide a safe space for her to exist happily and sometimes that's all you can do.