I personally love fiction that involve horses! With any story, I think it is important for the main character to overcome some sort of challenge and have personal growth.
I recently read a book that was based around horses, but it was clear the authors knowledge regarding them was minimal. I don't know what your experience is surrounding horses, but do your research. You can even use the details of your story to teach other people about horses.
Keep in mind that writing is more for yourself rather than other people. If you get caught up writing for the purpose of pleasing other people you will be disappointed. If YOU want to write a book about horses than go for it!