The boy grins and he rubs your human chest and then hugs you in a tights squeeze that forces your arms against your sides. When he lets go, you are startled to discover that your arms wont pull away - they seem to be stuck to your chest!
The boy doesn't seem to see anything wrong, and picks up the soft brush to start on your human torso. You pull at your arms again, getting upset. "I can't move my arms! They're stuck or something!"
"Oh, I'll fix that." The boy starts to brush your arms and sides, working down from the shoulders. Though you are bewildered by the trouble with your arms, the brushing distracts you. It feels wonderful, sending a pleasant warmth into your body. Twisting your head, you can just make out the lower parts of your arms as he runs the bristles across your hands.
Your fingers look odd, almost blurred. Curious, you squint to focus your equine vision better as the boy brushes over them again. This time you can see the shape of your hand blend into the sleek black horse hide. You blink as he pats the blank skin and runs the brush up to what was your shoulder. Your arms are gone!
"What are you doing!? I didn't want to lose my arms! Make them come back! Make them.. whinny!" The boy reaches up and brushes your throat once with the brush, causing your vocal chords to become equine. Your squeals and whinnies become deeper and more horselike as what was your human chest and neck merge and change into a normal horse's neck.
The boy gets up on the stool and starts to brush out your mane and the short black fur on your muzzle. You can feel your head collapsing, your face pushing out even more, becoming the massive head of a true Shire stallion. Why can't you run? Attack? Do something?
The boy pats your neck as he finishes brushing out the last traces of humanity from what is now a perfect Shire stallion. "You are under the animal control spells now - you are being forced to remain still and calm. Don;t worry - you're perfect and beautiful! A ten year-old stallion in the prime of life, able to mount a dozen mares a day!
He steps down and moves the stool. "I'll release you now. You should still understand me, at least for a while. Once you start mating, you'll gradually adjust to being a horse, so all of this will seem perfectly natural to you."
He points to the stable. "You can go out to the pasture and check out the mares now, or go to the empty stall on the left. That one is yours now."
You are suddenly free to move.