You got kidnapped by a crazy scientist. You were knocked out, and you were being experimented on to turn you into a centaur. The scientist cut off your lower legs completely, and he has replaced your lower body with the body of a horse—a strong horse.  

Your upper body has been enhanced. He used a drug on you that made your upper body more muscular and strong. You can be a male or female centaur, and you will have strong muscles regardless. You wake up in the beautiful farm pasture. 