Since you already have him looking more like a Clydesdale, you decide to go all the way with it. He looks dazed by the continued pleasure from your internal squeezing and doesn't react as his arms stretch out into powerful forelegs that clamp around your sides and his body swells out huge and muscular.
You feel him get hard inside you as his testicles refill, and gasp in renewed pleasure as he flares inside you again. You catch glimpses of his face pushing out, his neck lengthening, and golden fur spreading out across his skin. But then all you can think of is the mating itself, pure animal lust and instinct as the male empties himself into you once more.
It takes a long time to clear your head this time, and when you finally remember what you were doing, you discover a magnificent Clydesdale stallion standing next to you in your stall. His scent is powerful, and makes you feel safe and submissive. He is lipping at straw, and nuzzles you occasionally. But he shows no signs of human memory or intelligence - you have turned your former master into a normal animal - your mate!