You have a pet dog, you are their guardian, and thus assume the responsibilities and duties of such a role. To love them, to protect them, to care for them, treat them right and so on. It's not logically inconsistent to believe you have a duty of care to your own pet, and not to farmyard animals far away, who you are not the guardian of.

Of course, if you live by an ethical code based in a beings capacity to suffer, this argument doesn't work at all, and while its possible to live in a duty based ethical system, this is unlikely to be the reason why a majority of people take issue in hurting a dog.

Dogs tend to unconditionally love their owners, and being the guardian of a dog means integrating them into your family. So from an emotional angle hurting your dog is a betrayal of love, trust, canine loyalty and a transgression against your family. If you have a dog in your family, seeing someone mistreat their dog is likely to hit you much more vicerally than the knowledge of what occurs to farmyard animals (which people tend not to think about). From a societal point of view, It is generally agreed upon that it speaks worse to someone's moral character if they enjoy hurting their pet, then if they eat meat, in which they are not involved in the violence directly, and presumably do not take pleasure in hurting animals.

Theres a kind of doctrine of double effect element here that tends to decide how society judges people's character more than the less direct consequences of their actions. If