The mare grazed with the rest of the herd. Though she did not look it, she was the oldest by more than a hundred years, and all of the Clydesdales here were her offspring. Except for the curiously familiar new horse-man her horse-man was talking to. Her nostril quivered as the newcomer's scent changed, from a strong male odor to... her ears pricked up. that was her own scent! Confused, the mare whickered and watched as one of her great-grandsons covered what was now a huge, dark mare. When the horses had done mating, she started to amble over, only to whicker in surprise as her centaur and the new mare vanished.
Within a minute, the mare staggered and whinnied in panic as her neck and head started to expand. Her mind spun and her own scent was changing - to that of the centaur male! Memories lost over the decades of life as a mare began to return, and as the last pieces of the transformation completed, Derrick gasped as he found himself to be a centaur again.
"Whoa!" He stared wide-eyed as memory of the past 200 years paraded through his mind. So many foals, so many stallions! He twisted around, the dimly-remembered centaur body now his reality. There was a flicker behind him, and he turned to see Hephaustus appear with a grin. "Welcome back, boy! Then again, you just left a minute ago."
Derrick grinned. "That was... amazing! I was a horse, a mare for real!"
The older centaur nodded. "Aye, boy. I told you it would be an experience like no other. So, what now? Do you want to try life as a stallion? Stay a centaur? Go back to being a boy?"