Write a fanfic where NAME_1 are riding their horses in the forest. NAME_2 is riding a beautiful mare while NAME_3 is riding a massive very large and muscular virile breeding stud stallion. NAME_3's stallion is a true beast of a horse and is the pinnicle of masculinity and virility. He is known for his extremely large (even by horse standards) genitals and for being very amorous and an excellent stud. His testicles in particular are very large being even larger than soccer balls. As they are returning home NAME_1 are cursed by NAME_4. They are merged with their horses becoming centaurs. NAME_2 is merged with her mare having the mare's body below her waist while NAME_3 is merged with her stallion. They must now learn to adapt to their new bodies and figure out how to walk on four legs. They are 100% merged however and their lower horse bodies are now part of them and are considered their own bodies now.