"my centaur lower body!" Jenny finished as Jeff slightly turned his head and raised an eyebrow looking at her in curiosity.
"A centaur? Like you being a woman with a horse part?"
"Yup, and you will be that horse part!"
Jeff thought about it, he saw stallions as a powerful being in nature, he would love to be one, well, at least like 80% of it. "Let's do it!"
Jenny smiled, typing the changes on the computer. When she was done she asked Jeff to remove his clothes while she did the same, they giggled watching each other casually naked as obviously wasn't the first time they saw their birthday suits.
"So what should we do now?" Jeff asked approaching Jenny.
"Stick your butt with mine!" Jenny chuckled. She knew what was about to happen including its details, while Jeff had no clue what his girlfriend's idea of a centaur lower body would be, for a moment he began to doubt.
Now Jeff slowly touched his beloved one's rear with his butt, "is this supposed to do someth-" he didn't finish as he felt their asses combine, like putting two masses of clay together. "Umm babe, what if we do something else?" He nervously smiled.
"Nope! The changes are done and we can't do nothing about until the duration expires!" She bit her lips as their legs merged into one pair, "also this feels really good."
Jeff couldn't say the same, the process wasn't being painful or bad, but it was really odd and scary from his perspective. Dark brown fur began to expand from their feet to their waist, after their feet morphed into hooves, later the fur began to grow across Jeff's body which began to lose balance and eventually fell, standing on the floor with his hands while the other half of his body was attached to Jenny's rear. "This is... Weird" Jeff whispered to himself but now he seemed to secretly enjoy their transformation.
"Sorry sweetheart, but you signed for this. Don't look at just the bad part of it, imagine all fun stuff we could do! Run really fast, the strength we would have and..." She began to have some questionable thoughts, images of... Horses? Coming to her mind, in a lewd way.
"Oh God" Jeff exclaimed while sweating and his heartbeat getting faster, the fur already covered all of Jeff's torso, now going to his arms and head, his hands suffered the same fate of their feet of changing into a pair of black horse hooves. "I feel powerful now!" Jeff thought, Jenny somehow knew that he was enjoying their fusion as much as her.
Later their new horse legs reshaped its structure to have a more accurate horse anatomy as Jeff's body grew in musculature, then Jeff began to taste something tasty, a strange fluid coming from his mouth, it tasted like when he gave an oral to Jenny, it could be...
Jenny moaned, touching one of her breasts in excitement feeling Jeff's final change. The fur finished to expand through all of Jeff, and they already had the form of a centaur with Jenny as the human part, the last touch was to change Jeff's head glued to her horsey rear. His head was slowly swallowed by the body, his mouth and nose gained a black color, his nostrils merged into a single hole while his mouth turned into a vertical opening, Jeff knew what was happening and couldn't deny that he loved it. There was almost no trace of Jeff as a human, just his eyes that were shut by being covered by Jenny's horse butt hair, before his mouth finished to turn into the black, deep and long vagina of a mare he said his last words. "I love... You!" His transformation was finished as his nose successfully turned into a mare anus.
"I love you too honey!" After she moaned a big stream of fluids came from her equine sex organ, leaving a puddle on the floor after that wonder orgasm her boyfriend gave to her.
Jenny saw herself in the mirror as the beautiful creature she was, she turned around to see her centaur lower half she promised to Jeff, noticing that her tail was black matching Jeff's hair instead of being blonde like her. "Thank you so much for this sweetie, I know you can hear me, so let's have some fun!"
As Jenny said Jeff was still there even if his existence was reduced to a horse pussy and butt hole since Jenny had control even in the horse legs, but he could still see, hear and feel through her centaur rear, he wondered what plans she had in mind.