Equine massage therapy is taught in one week. If you want to make a career of it, then you might consider going into veterinary medicine and adding massage therapy certification to your credentials. I personally would not let anyone manipulate my horse's tissues who was not a licensed veterinarian. Many vets go on to train for certification in chiro and massage, and they are the only ones truly qualified to practice these specialties. I strongly discourage anyone from completing one of these courses and then hiring themselves out to practice on horses, and I hope to see this better regulated in the near future. Unfortunately, at present, anyone can spend a week in one of these schools and then call themselves a massage therapist. The certification is given by the school, and means virtually nothing.
But you can make money at it, as long as someone out there is willing to pay you. Unless it becomes better regulated, you might be able to make decent money doing it.