I wonder if the dog....

might not have even existed. The whole movie comes off as an existential treatise on humanity in general, the dog most likely represents Dugan's frustrated sexual mind, notice how it reappears when he gets in the teacher's car, jumps in the backseat to go for a ride with them. The teacher doesn't even seem to notice, only Dugan does. In fact, no one ever seems to notice the dog stealing the bikinis, or the topless girls running down the beach. The bikinis also seem to be hoarded by the dog in a part of the beach where obviously no one goes. I think this movie is a perfect movie to see after reading Sartre's Being And Nothingness, or maybe as a double feature with The Tree Of Life.