There are a few articles that I’ve listed below from PUBMED. Like many supposed therapies, most evidence is completely anecdotal. In my experience with kids with autism in using this therapy is that it makes the kids very happy and is a welcome break in the often monotonous character of other therapies, which is probably somewhat therapeutic in itself. It is seen to be efficacious because the kids often become more talkative when they’re doing fun, physical activities. I’m not sure about reported benefits for other disabilities.
Horseback riding as therapy for children with cerebral palsy: is there evidence of its effectiveness?
Phys Occup Ther Pediatr. 2007;27(2):5–23. Review.
Does horseback riding therapy or therapist-directed hippotherapy rehabilitate children with cerebral palsy?
Dev Med Child Neurol. 2007 Jan;49(1):68–73. Review.
Abstract The effect of hippotherapy on ten children with cerebral palsy.
Pediatr Phys Ther. 2004 Fall;16(3):165–72.
Evaluation of therapeutic riding (Sweden)/hippotherapy (United States). A single-subject experimental design study replicated in eleven patients with multiple sclerosis.
Physiother Theory Pract. 2005 Jan-Mar;21(1):51–77.