Try some hand dyes and I agree stonehenge fabric has a lot of texture.
I love horses, used to ride as a kid and now am lucky enough to work one morning a week with a client who attends Riding for the Disabled. She loves riding and I love leading her around whilst she does her lesson. We also spend a lot of time bonding with the horses and ponies, grooming and feeding as well as some arena work (she did lunging last week).
All the best with your idea and don't forget to show us.....we love pictures.