As your face pushed out into a muzzle and your transformation finished you felt thirsty, thirstier than you had ever felt before in your life. You could smell milk nearby and you trotted forward on your now four legs, still getting used to them as you nuzzled the mare's udder, something at the back of your mind told you that this was weird or wrong or something but you didn't care.
You took her teat into your mouth and started to suck at it which caused her to let out a light whinny. She smiled a little and looked back at you as you felt the milk running down your throat as you drank from her and your mind slipped farther and farther away. The images of your parents were replaced with the same horse you were nursing from, thoughts of friends and school were replaced with memories of playing with your sister and other foals in the pasture.
When you finally finished you backed off as the last of your humanity slipped away. You looked up at your mother who nuzzled you gently. "My handsome little colt," she nickered. "You're going to grow up to be a handsome stallion stud, just like your daddy."
You smiled a little and then went over to where your sister was standing and began to graze. You looked up briefly when you heard humans calling out on the other side of the fence as if they were looking for someone. They looked strangely familiar, but humans all looked the same to you and you didn't care as you went back to grazing.
You were a horse now and forever.