Listening to their conversation, the dad was saying that cave paintings were either used to summon the animals the cavemen were hunting, or for voodoo shamans to open a hole to another world. Apparently, that survived into modern graffiti, meaning the painting of a giant dog was actually a real giant dog. Similar to the beginning of the first Paper Mario, where you pass a sign on Goomba Road that says "Beware of Goombas!" and the picture of the Goomba on the sign peels off and tries to fight you. Anyway, the dog thing was either hungry or offended by the father's words, probably the latter.