I'm planning to have my senior pictures with my horse too, and I plan to actually have an equine photographer do them. I suggest you have a photographer that has experience with photographing horses, because they know how to make them look good and what poses shows you both off the best.
The biggest thing I suggest is to wear something pretty timeless. A simple dress is fine, or a nice t-shirt, or a simple blouse. But don't do something really trendy right now, as tempting as it may seem. You'll look back years later and laugh it how ridiculous you look!