Animals can add a much needed lightness to a dark book, giving the reader a break from the heavy emotion and possibly bringing a smile. The crazy, natural things they do are just plain funny, and funny can and does happen at the worst of times. Make them laugh, make them cry, they say.
Animals can help with both.
Animals can reveal character. A hard-edged hero, showing affection to a dog or a horse, can tell the reader that he isn't quite as hard as he would have people believe. On the other hand, a man ignoring an animal or treating it badly can spell villlian without even saying so. Toddy was inspired by my sister's poodle. Toddy had been given to Bella Duprey, a Barbary Coast courtesan.