The laws that allow livestock owners to kill attacking dogs do not usually apply when it's another dog that is being attacked. So, a dog owner who injures a dog in an attempt to protect his own dog may still have to pay for the injury, or answer a cruelty to animals charge. It will be up to a judge or jury to decide whether the owner acted reasonably under the circumstances. For example, a New York man was found innocent of a cruelty charge after he shot and killed a dog that charged into a family picnic, scattering the children present, and attacked his dog. (People v. Wicker, 78 Misc. 811, 357 N.Y.S.2d 597 (Town Ct. 1974).)
Even though the owner of a dog that is being attacked may not legally be allowed to kill or injure the other dog, he probably can sue the other dog's owner. Remember that legally, a dog is its owner's property, and the owner can sue for property damage. In a slight majority of states, dog owners are financially responsible for all property damage their dog causes, in most circumstances. In the rest of the states, the injured party must show that the dog owner knew, or should have known, that the dog was likely to cause that kind of damage.
A real-life example: In 1919, a four-and-a-half-pound Pomeranian was being walked on the streets of San Francisco when it was attacked by an Airedale terrier. The attack was fatal; as the court put it, the little dog "crossed to that shore from which none, not even a good dog, ever returns." Its owner sued the Airedale's owner for $1,000. (This Pomeranian, according to the court, was "regarded in dog circles as possessing the bluest of blood.") The Airedale's owner was found liable. (Roos v. Loeser, 41 Cal. App. 783 (1919).)
In a more recent case, the exasperated owner of a purebred beagle sued his neighbor after the neighbor's dog got loose and impregnated the beagle—for the second time. Because of complications of the second pregnancy, the beagle had to have a cesarean section and later, a hysterectomy. The court ruled that the amorous dog's owner was clearly negligent and liable for the beagle's $851 veterinarian's bill. (Kurash v. Layton, 251 N.J. Super. 412, 598 A.2d 535 (1991).)
Penalties built into state law. In Maine, the owner of a dog that kills or injures another pet may be subject, under state law, to a $100 forfeiture in addition to whatever damages the pet's owner suffered. (Me. Rev. Stat., Ann., tit. 7, § 3962-A.)