When petting your dog's belly, you might notice they lack a navel similar to your own. While mammals do have belly buttons, they're a little harder to find.
Since mammals are gestated inside their mothers, they are all born with umbilical cords. When they emerge, the mother chews off the cord with her teeth, leaving a flat scar that is less noticeable than a human's navel. Often these are harder to see and obscured by hair. As a result, your pets might not have the most obvious belly buttons. Chimpanzees and gorillas have ones similar to a human in shape, but they do not go in or out.
Of course, there are always exceptions: platypuses and marsupials do not have belly buttons. Platypuses lay eggs, so there is no umbilical cord. Marsupials' umbilical cords usually fall off while in their mother's pouch, and a scar never forms.