If you want to provide therapy services you will need to decide if you want to provide mental health services or more medical based services. Mental health can be provided by a social worker, psychologist or psychiatrist. Medical based services for hippotherapy are provided by occupational or physical therapist or speech language pathologists. For all these professions it's great to have a horse back ground, but you will also have to spend time becoming a competent professional in your field, prior to providing specialized services incorporating horses. In the US, there is no formal designation for "horse therapy" this means that the professionals listed above are providing counseling or physical therapy and they incorporate horses into their practice. This terminology is very important as misrepresentation of what services you provide can be considered fraudulent or void your liability insurance.
You could also chose to provide recreational or sport services to people with special needs through therapeutic riding or driving by become a NARHA or CHA certified riding instructor for persons with disabilities. In these cases you are not providing therapy, but a recreational or sport outlet for people who want to become riders, but need special adaptations.
I have interviews with professional in the Equine assisted activities and therapy field on my blog as well as downloadable presentations dealing with terminology in this field. Hope this helps.