There were all kinds of animals in Red Dead Redemption, including dogs. They could all be killed and even skinned. I think there was even an animal (not the sasquatch or whatever it was) that, if you killed enough of them, they went extinct in the game. Of course, the killing of a dog in GTA V will be for no reason (maybe self defense if the dog is attacking you), while most of the time killing animals in RDR resulted in rewards that could be used for trading. As said before, Call of Duty has had dogs and there was no problem with them, so I think the inclusion of dogs really comes down to if Rockstar feels like adding them or not.