I used to do a fair bit of equine acupuncture then moved interstate and subsequently returned to treating people.
Equine acupuncture is very rewarding however has many unique challenges.
Firstly, your patient is quite likley to bite and kick, even all at once and in the one move.
Secondly, if your using acupuncture needles rather than LLT (Laser) the horse often thinks these are flies and twitches them out leaving you to literally find a needle in a haystack... Bear this in mind if utilising copper handles.
When you are treating horses that may be "worth" half a million or more its also worth reviewing your insurance to make sure it's all in order.
On the upside, with the extra 4 vertebra and the weight of a jockey, it not at all surprising that that many horses get back pain. They generally respond well to acupuncture, infact often much faster than humans do.
I am not presently working with horses but maybe soon.
Ps, have you looked into equine chiropractic (other uses for a tennis ball), massage or equine dentistry, as these can be good synergy.