Belly buttons are few and far between in the animal kingdom, but the majority of mammals have them. Since placental mammals are all gestated inside their mothers, they are born with their umbilical cords still attached. Once delivered, their mothers typically chew through the cord, which tends to leave a flat scar or small bump that is much less visible than a human’s belly button, with the animal’s hair often obscuring their navel even further.
There are mammals that prove an exception to this rule. Platypuses lay eggs, so there is no umbilical cord, while marsupials do have umbilical cords but these usually become detached when inside their mother’s pouch. As they are still very small at this stage, a scar never develops into a visible navel.