A tourist goes into a bar, and there's a dog sitting in a chair, playing poker
A tourist goes into a bar, and there's a dog sitting in a chair, playing poker. He says, "Is that dog really playing poker?" And the bartender says, "Yeah, but he's not too good. Whenever he has a good hand, he starts wagging his tail."
This is another bar joke. In the joke, apparently the dog is smart enough to play poker, but is still a dog, and so he wags his tail as a sign that he is happy. If a dog were smart enough to play poker, it should also be smart enough to try not to show when he has a good hand.
Perhaps this is a "dogs are dumb" joke.