When casting for animals, I like to do research on deities that particularly relate. For instance, when my dog was ill, I performed a healing spell and asked Artemis for assistance and blessing, since one of her animals is the dog. I discovered the cause of his illness in two days time, my dog recovered quickly and has been totally healthy ever since. I honored Artemis by burning bay leaves and reciting an ancient hymn of hers in thanks for her help.
Animals can gain the same benefit from any spell utilized for humans, but I find that they are a little more difficult to get results from unless you include the aid of a spirit/deity that watches over that animal. My opinion on this is that animals are guarded over by spiritual forces that are more stubborn to comply unless approached/reguarded first. When influencing other people, we are on equal terms, being humans ourselves and just as guilty of straying from nature. But to touch an animal with magick is to influence an "innocent", since animals are very instinctual and closer to their own natural ways than we are. We avoid and deny much of our natural instincts and carnal ways and harm nature more than help it (humans as a whole).
Off the top of my head, Posidon and Apollo were deities over horses. Posidon in myth created the horse, and of course Apollo rode them across the sky. If you don't like the Greek pantheon, you may wish to look for a deity that fits the culture/religion you feel most comfortable with. I always enjoyed Greek gods and goddesses because I feel they are more approachable than others.