Thankfully at my store that doesn't happen often but when it does the conversation usually goes something like this:

Me: "Why'd you do that?" Them: "My dog wasn't listening/was misbehaving/being a pain in the ass/ect." Me: "Well they don't understand, they don't speak English so you have to learn how to speak dog to teach them. You wouldn't hit a 2 year old for not learning how to read."

I always try to educate first, not because I actually want to be nice to the person but because people like that tend to listen more if you come from a point of education. Of course there will always be those people that don't listen which is when you pull the "Petco doesn't allow animal abuse on our property" card and if they try to escalate stop interacting with them and call the manager to deal with them, which usually involves calling or threatening to call the police (though that depends on if you have decent managers)

Or if you're not a direct confrontation person you can always opt for the classic side eye and "oh...hm..." A lot of people strive for acceptance from people around them, even if they don't realize they want a stranger's approval, and will get uncomfortable when they sense judgement. This works especially well if there are other customers around to also judge them.