Children's brains do not understand the nuances of punishment. Even if he hadn't hit another dog before, I guarantee you that he was hit before. Hitting (of ANY form, spanking included) your child vastly increases the chance that they will act out violently themselves, because it teaches them the lesson that when someone or something does something you don't like, you can and should use violence against that person or thing. Children who are spanked become more likely to strike fellow students once they join school, and are more likely to beat their own spouse and children once they have a family. You are literally re-wiring the child's mind to use violence to express displeasure. People who are beaten as children wind up beating their children, and children who are beaten grow up more violent than children who are not. Furthermore, you teach them to ignore any form of non-violent authority, so they are more likely to break rules at school and break laws in general, because now violence is the only form of discipline their brains understand and respect. Studies have reaffirmed this connection again and again.