You follow the boy into the big barn, and he opens a large stall. "Go on in - I'll leave the gate open, I just have all my grooming stuff in there."
The stall is clean, with fresh, sweet-smelling hay and a large window that lets in air and sunlight. You decide to follow instructions a good grooming does sound nice.
The boy grins and opens a box in the corner, pulling out a couple of brushes, a scraper of some sort, a big squeeze tube, and some jars. Then he pulls out a stool and puts it next to you. "I'll start with some basics first. Hooves need to be cleaned so you don;t get infections and sore spots." He moves to each of your new hooves and pulls them up, using the scraper to clean out dirt and other stuff that has packed into the hollow inside. It feels weird, but not bad. When he is done, he opens a jar and smears a whitish cream over the outside that soaks in and leaves your hooves shiny and smooth-looking.
"Now that you have horse hide, you'll need to be brushed out to stay clean. You just can't reach everything yourself." He starts with a firm bristled brush, which feels wonderful. It's like a massage, soothing and restful. Then he switches to a soft brush that makes your coat is glossy and smooth where he works. Curiously, you feel a bit more massive as he grooms you, as if you are growing muscles.
"Now for the fun part." The boy grins and picks up the tube. "You'll enjoy this." He squirts some clear jelly into his hands and then crouches down under your belly. You feel his fingers rubbing your heavy sheath, and understand what he is talking about. He seems to know just what to do, for your horse cock emerges and starts to throb erect as he smears the gel over it, working his fingers up into the thick folds of your sheath. "Horses get the natural body fluids dried up inside their sheaths - it's called smegma, and it needs to be cleaned off every now and then. Plus, you can't reach back here to feel yourself, so I figured I'd help you out some."
As a human, you'd be shocked at having anyone handle you like this, but now you find it intensely pleasurable. Your shaft pulses out as he rubs the sensitive flesh, swelling larger and longer. Twisting around the other way, you can just make out your hindquarters and see the impressive sexual equipment that is now yours. The boy rubs some of the gel on your scrotum, and you feel a pulling and pressure - and then realize that your testicles are getting even larger! When the boy finally stops, you have balls the size of large oranges in a smooth black sack, and liquid is dribbling from the flared tip of your penis.
The boy uses both hands on your shaft now, and you feel yourself harden. Your shaft arches up toward your belly, the oddly flat tip flaring out like a mushroom. You can feel your testicles pumping, and then shudder as liquid spews from the tip and shoots out between your forelegs. The sensations make you whinny in pleasure, and the boy masturbates you into two more ejaculations before he stops.
When he steps out, he has a handful of thick, whitish liquid - your semen! Moving around to your front, he smears the sticky stuff on your human chest and rubs it into the skin. Your nostrils are full of the musky odor that your brain is telling you is your identity. As the boy smears the semen in, he grins up at you. "You're doing great! Now that I've matured you and boosted your sexual abilities, we just have to finish up on top.
Matured you? Actually, you do feel more massive, more powerful. Mature - that means getting older. Something about that bothers you, but new sensations of warmth and pulling in your chest distract you from that.