That’s a mature male tarantula. You can tell by the hooks on his front legs which he will use to keep the female’s fangs at bay while he mates with her. The reason his abdomen is so thin is because, after they mature, males typically give up on eating and instead just search for a mate. He will likely be dead soon, either because he successfully mates with a female and she manages to grab him as a snack before he can flee, he dies of old age, or a predator picks him off as he wanders around.