It doesn't matter whether you use horses. It doesn't matter whether you use ABA, Horse Boy Methods, PECS, Sunrise, Play Therapy, Floortime, or a cocktail of all of them if you don't get the environment right - forget it.
Because kids on the autism spectrum have a hugher anxiety level than the rest of us and this is exacerbated by bad sensory stimuli plus they just aren't really motivated to communicate so - if you crate an environment, in your home, you stables, your back yard, that is sensorily healing, the child can - at long last 'hear and see you' because all that horrible white noise has finally quietened down.
So - here is the checklist.
No florescent, no cigarette smoke, no strong perfume, no industrial noise (vacuum cleaners count - do that when hes out of the house), not too much plastic but more wood, wool, antler, bone, stone - natural stuff that feels good to the touch.
Make sure there are plants in the house, natural light int he day and oblique softer light at night, small animals (GET A DOG); play equipment, (THEY NEED TO MOVE MOVE MOVE IN ORDER TO LEARN), and grass/sand/woodchips where they can run barefoot if you don't live in the country,any decent city park will do...
And finally a human environment of "YES' - try to eliminate dangers so you can say 'yes' to pretty much everything - that way you prove you're worth the hassle of trying to communicate with 'all this wont make it less easy for your kid to cope with 'real environments.
It allows him/her to create more positive neural pathways in the brain that, over time, make dealing with the 'real world' stuff easier.
There's science behind it too: http://www.reuters.com/article/2014/05/03/us-health-autism-idUSKBN0DJ0H020140503