Fritz and the Beautiful Horses by Jan Brett. Your daughter might be past simple counting books, but One Horse Waiting for Me by Patricia Mullins goes up to 12, rhymes nicely, and has stunning illustrations. And I see that Susan Jeffers was already mentioned, but she really does have a ton of books for horse lovers. We have an out of print version of All the Pretty Little Horses by her that’s somewhat dated, but quirky and very imaginative.