Hello there! Perhaps I can help. I am an Occupational Therapist and have acquired special training and certifications in the area of Hippotherapy, & Therapeutic Riding. The majority of my caseload was children with disabilities, mostly on the Autism spectrum.
The American Hippotherapy Association has formal literature regarding providing Physical, Occupational or Speech Therapy with the help of a horse (using the movements of the horse to be therapeutic modality during treatment sessions). Hippotherapy is different than "Therapeutic Riding" which is the act and end goal of teaching a child/adult with a disability 'how to ride'.
There are measurable, physical, social and emotional benefits of both, however, treatment of the child/adult based on the disability/symptomology can be achieved more with Hippotherapy as it can only be practiced by trained and approved Occupational, Physical and Speech Therapists.
PATH International (Professional Association of Therapeutic Horsemanship International) --formerly known as NARHA, provides accreditation of centers/organizations that provide Therapeutic Riding AND Hippotherapy. Both organizations are very valuable resources for researching benefits of equine involved works with people with disabilities.
I hope this is helpful!