"Wake up, centaur Mike."
Mike's eyes snapped open. The accursed saddle could even force him to obey a command to awake, it seemed.
Nia stood before him, appearing somewhat nervous.
"Father is drunk again, and asleep in his room," she explained, "Before he passed out, I told him that the fields are very dry, and that it would be easier for you to plough them if there was rain...or if you were stronger."
Mike looked at her with interest. She pulled a familiar bottle from her pocket. "Father stumbled around a bit, and then told me to give you this." She held it out in her hand. "Drink it, centaur Mike. But drink it while you make love to me!"
Mike rose, and his powerful horsecock became hard. He took the offered bottle with one hand, while helping Nia remove her clothes with the other. She ran her hands up and down Mike's powerful torso, and sunk to the floor, between his legs. "Fuck me, big man," she purred, "Fuck me, drink that potion, grow huge, and fuck me some more!"
Mike lowered himself onto Nia, as she guided his dick to her pussy. Its thick hardness filled her, and she moaned in delight as Mike began thrusting into her. He drank the potion at the same time, and felt that familiar acid tingle spread throughout his body, followed by a strange numbness.
Mike's already large muscles twitched and swelled, becoming monstrous in size. His entire body grew larger, as did his cock, buried within Nia. The girl screeched softly, delighted by the new sensations the larger dick produced, but terrified of it as well. Mike's thrusts became more powerful as his strength increased. Nia grasped his huge, swelling muscles, murmuring with appreciation, urging him to continue growing and rutting. Mike felt the saddle tighten around him, but still it remained firmly attached to him, although its straps were stretched to their very limits.
For the next hour, Mike used his massive new body to pleasure Nia in whatever way the slutty farmgirl desired. His upper body was larger and more powerful than the biggest, strongest bodybuilder. His lower body would make a Clydesdale look scrawny by comparison. His cock was in a league of its own, yet somehow Nia managed to take its impressive girth. Most of all though, Nia loved to admire Mike's freakish musculature. Finally she retired for the night, letting Mike sleep a few precious hours until morning.