For my purposes, I've only listed groups of creatures that produce (or produced) fertile hybrids with each other

THE DIFFERENT BREEDS OF DOMESTIC DOG; HUMANS SHOULD STICK TO THEIR OWN RACES, BECAUSE US ANIMALS STICK TO OUR OWN KIND! THE DIFFERENT TYPES OF GIRAFFE; DOMESTIC DOGS, WOLVES, AND COYOTES; POLAR BEARS AND BROWN BEARS; RED DEER AND WAPITI ELK; MODERN HUMANS AND NEANDERTHALS