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!
Riders aren't 16 and pregnant. Riders are 16 and arthritic.