Haha, interesting question. Haven't thought of that. In many fanarts, they have. I believe they also have belly buttons in the show, it's just not visible because their fur, manyoya covers it. I mean, equines are mammals, therefore if we approach the swali from an anatomical perspective, they must have a belly button. :)
Um no, farasi and ponies are animals, non human wanyama would be weird if they had one. Non human wanyama are not attached to an umbilical cord, I've never seen any non animal be attached to one.