Atoms and Void

If you want to incorporate desensitization into a ponyplay scene, you need to pick a thing that the pony is genuinely uncomfortable with (could be anything from spiders to bad textures to tickling to nudity, whatever kind of scene you wanna play out).

I'm gonna assume the horse in question is already target trained and clicker trained.

What you wanna do is show your horse the uncomfortable stimulus and ask it to interact with it. With theriform horses, you can often coax a horse into this by placing the target near the uncomfortable thing or by asking the horse to target the uncomfortable thing itself. Every time your horse interacts with the thing, you click and reward.

If your horse's fear of the thing trumps its desire to touch the target, you can practice operant counter-conditioning. You will have to force your horse to interact with the uncomfortable thing, but never to a point where the horse's threshold of stress is crossed. Every time you force the uncomfortable interaction, you simultaneously ask the horse to touch your target and then reward. Theriform horses generally love target training and are extremely food motivated, so they quickly come to associate the bad thing with the good thing. Eventually, with enough persistence, they should lose their negative associations with the uncomfortable stimulus.