I would only let a suitably qualified person treat my horse, especailly if I'm paying for it.  I have an equine physio who uses ultrasound and pulse magnet therapy but she is a fully qualified veterinary physio, and this reassures me that a) she knows what she's treating and b) she knows how to treat it.  At the end of the session she produces a full physio report and also sends a copy on to the vet.  You would likely need insurance, third party but with a clause in it that allows you to treat animals, just in case you unintentionally make something worse.  I think you should look into getting some training before you start out, just to be on the safe side.