Just take a look at the differences in shape, lighting and even the nose/snout. Take the canine for example, his ears are slightly turned outwards and the lighting above his nose makes it look like it's somewhat longer than the rabbit his nose.
Keep in mind that with most sprites we have a slightly top down view, so the hairline is going to be relatively closer to the eyes than we would have in a straight down view. And that although you block out the hair in smaller shapes you still shade the hair as one shape. Because individually shading each group will result in a poor image.