All mammals have bellybuttons, but they are usually (1) small, and (2) covered with fur.

kitty

http://farm1.static.flickr.com/36/121828967_d531d37139.jpg?v=0

whale

http://www.voluntours.co.za/joomla/index.php?option=com_zoom&Itemid=116&page=view&catid=4&key=5&hit=1

BBFluff may be a human only thing, though.