Grand Theft Auto 5 is the latest, and arguably greatest incarnation of the GTA series. It's set to put players in the shoes of not one, but three protagonists—a GTA first. The mechanics of the three characters have been described by Rockstar as thus: you'll be able to switch between any of the three at "nearly any time", each of whom has their own personality, skill set, and group of friends—or missions, to deal with. When you're in control of one character, the other two will go about their own business until it comes time to do a main story mission.
In any case, one of the three main characters, Franklin, the repo-man, has with him a Rottweiler named Chop. It's no indication that there are going to be pets in the game, because Chop is special and he follows Franklin around. One thing we noticed about Chop is that he (or she?) can be seen sporting different collars in the game's 2nd trailer and a newly released screenshot in which he's riding in a cadillac with Franklin. He appears to be wearing a pink collar in the new screenshot, different from the yellow collar he had in the video.
This brings to mind the possibility that Chop's collar can be customized as well as the outfits the characters wear.
It's unknown whether Chop will only follow Franklin around during story missions, as he can be seen chasing people down in what is presumably a cutscene during the 2nd trailer—or whether Chop will be at Franklin's heels at all times throughout your control of the character, and how useful he'll be during gameplay. I just don't see him taking damage and dying during combat given his apparent role in the game's story.
We'll find out more about Franklin and Chop as the game nears release. It's set to come out early next year for the PS3 and Xbox 360. No version for the PC or Wii U have been announced as of yet.