There isn't anything wrong with booking a single bodywork session for your horse, as long as you understand the difference between a single appointment and ongoing care with an equine wellness professional.
The real benefits come when an equine therapist can see your horse regularly, or at least for several sessions in a row. The length between appointments can vary quite a bit, based on the therapist and the individual case. Having an open conversation with the therapist about goals, expectations, and budgets is the best way to determine what fits you and your horses' needs.
My first session with a horse can be quite lengthy, often two hours, because I spend a lot of time observing, palpating, connecting with the horse and their human, and asking questions about their histories. Bodywork is included in these sessions (if deemed appropriate for the horse), but we need to build slowly upon what we start in these first-time sessions - especially when working with modalities like craniosacral, myofascial therapy, and nerve release work. These are very deep, profound techniques for the nervous system. Doing too much, too fast, is absolutely possible.
While I want to see every horse that is brought to me (because even a single session can be beneficial) I am hesitant when someone says they want to book one appointment as a treat for their horse. It is definitely possible to have a relaxing, feel-good massage session with a horse, without all of the observations and palpations and deeper work. Unfortunately for me, that idea of "you can't unsee it" definitely applies here. Once I feel it, I can't unfeel it, and I can't turn off my inner therapist to give a one-off, simply feel-good session anymore.
It's the same reason why an athlete wouldn't book a hot stone massage before a big game - sports massage exists for a reason! A therapeutic massage likely isn't the vibe you're going for on a spa day, and you wouldn't call the spa to help you with a muscle injury. Many, many kinds of therapies exist to meet all of our needs.