"So," Derek said, "you've got complete control over my body."
"Except that I have to be sure you can still speak English and aren't an amphibian. Don't worry, I'm going to be sure to honor that arrangement."
"That's good, because I don't have a choice at this point. But, you know I never worried about it for a minute."
Sonya smiled, "We want this to be fun. But, let's start with something easy we can reverse quickly. Derek, grow a second thumb on each hand." And they instantly grew adjacent to his existing thumbs.
"Oh, wow, they're real! They feel natural and odd at the same time. But, enough testing. You know you had to be thinking about something more than thumbs."
"You're so right. Let's make this a new experience. Derek, your second thumbs now disappear and you turn into a cow-taur. Below the waist, you're basically plugged into the foreshoulders of a cow bred for milking."
Derek suddenly grimmaced, then his lower half dropped to the floor and exploded out of his pants. "Ow! Oh, that hurt." He looked at himself, and he was now a cow below the waist."
"I'm SOOOOO sorry, I just didn't think to have you get stripped first. Are you ok? Maybe we should both take off the rest of what we're wearing." With that, Sonya stripped completely, leaving Derek in his top, his shoes having fallen away from his hooves, and his socks still on his rear hooves.
"I'll be ok. Don't worry about it. But, you know I'm a girl down there now, right?"
"Exactly what I had in mind, and trying to give you ideas for when it's my turn. Are you ready for more?"
"Always. You know I like girls. You're trying to get me to turn myself on?"
"Oh, that's certainly part of it. Your human half will now become completely female. Directly below your first set of arms, you'll grow two identical sets for a total of three sets of two arms. Directly below your breasts, you'll grow two identical sets of breasts. Your breasts will all grow to a DD cup size, and you'll be able to easily give yourself unlimited orgasms by playing with your breasts. The nipples on your top pair of breasts are long enough so you can suck on them easily." As she stated them, the changes happened, right down to placement of 5" long nipples on the top pair of breasts.
"This is going to be an awful lot of milk. Good thing you've given me 10 limbs to work with."
"Before I go any further, let's see if this works the way I hope it does." With that, Sonya walked up to Derek, whose cow half was still essentially reclining on the floor, knelt down, and took one of his top nipples in her mouth and started slowly to suck, and to play with the nipple gently with her tongue.
Sonya was rewarded with an orgasmic moan from Derek that was half-scream, and all at a higher pitch than Derek's voice was ever able to achieve before. "OHHH! OHHH! YESSSSS! So good. So good. UHHHH! Moremoremore. YESSSSS!!" Very quickly, Derek started to shudder, and his muscles turned to jello.
"Seems I got you your first female orgasm, and probably in less than 30 seconds. Are you happy with the changes?"
"You have to ask? YES!"
Sonya grinned, entirely happy with the outcome so far, noticing Derek's six hands slowly moving toward his six new breasts. "I'd say we keep going until you get a hard-on, but I think you might actually try. Roll on your back, I have something else I'd like to try." With that, Derek did roll on his back...both of his backs...and Sonya continued. First, she climbed on top of Derek just below his forelegs, and leaned forward. She used her hands first to hold one of his middle breasts, putting that nipple in her mouth, and she began to suck on it a bit vigorously. Then, she moved her hands to Derek's lowest set of breasts, and started to massage them. Derek's hands started rubbing Sonya's back as well as his own breasts and nipples, then moved frantically to both their sides, and Sonya's breasts. Derek was very excited, and didn't entirely know what to do next. But, everything he tried seemed right. Sonya started to grind into Derek's lower chest, and was getting herself very much excited. After a few minutes of this, Derek was almost delirious, with no experience how to handle a female orgasm, or, certainly, how his cow body would get worked up. At that point, Sonya pushed herself up. "Keep playing with yourself, I'm going to try another position." Sonya got off of Derek, turned around, and got back down. This time, she was standing above his forelegs, facing his back end. She lay down on Derek, then took a cow nipple in her mouth, started sucking, and moved her hands over an impressively large milk bag, nipples and all. With this, Derek couldn't stop bucking, screaming, shaking, and gasping for more breath as wave after wave of continuous orgasm hit him. Sonya just kept doing this for about five minutes when she felt several feminine hands suddenly upon her.
"Now, it's your turn," Derek said. Sonya kept fondling Derek and sucking his cow nipple. Derek reached down and held Sonya's hips. Thanks to multiple hands, he started stroking her pussy, already very wet from the exciting stimulation she was already getting. Then, Derek started putting fingers in her. Three...four...not nearly full somehow...six...seven...eight.
"Too much, slow down, I love it but you're going to split me in half!" Without being able to really see, Sonya then reached forward and was able to reach Derek's cow pussy. The reaction was immediate and obvious. Sonya, with experience, was a little gentler and slower. Yet, after a minute, she reached in further, and had her entire hand inside Derek.
But, Derek still had a firm grip on Sonya. After orgasming from what Sonya's hand did to his newly minted pussy, Derek pulled her back...back from Sonya's perspective...and placed her pussy directly over his mouth. And he started to eat her out with total pleasure. A mouth on her pussy, sucking and licking, two hands on her hips, two more hands on Sonya's breasts, and two more wandering over the rest of her body, Derek was going to make Sonya melt. He felt the shivers and shudders of orgasm, and kept going. Sonya's heavy breathing and her body's reaction are all that Derek heard from her for nearly twenty minutes. She was nearly limp from the moment this started, and couldn't help but to just enjoy the ride.
"Derek, ohh, Derek, please. Let me come up for air."
"I was starting to wonder if you could still speak, even if I'm the one with the collar on."
"I c...can still...still speak if I can catch my breath. And I think the collar is a success, don't you?" Sonya struggled to her feet, getting up from Derek.
"Well, it's at least half a success. Are you ready to try it on?"
"I'm ready, but we might want to get something to eat before we do much more. We're burning a lot of calories in here and I'm really parched."
"One thing though before we trade places. Cows may be great for milk, but can you make me a traditional centaur before we trade places?"
"Sure. Derek, where you have a cow below your human waist, it will turn into a horse now and you'll be a centaur, with no other changes to your human half."
"I may never get used to that feeling, but I'm already starting to associate it with you, and with wonderful things we're going to do. And you know I'm still a girl down there, I assume."
"Well, Wonderful, let me take that collar off of you now. And you can put it on me. And, yes, I know you're all girl down there, and I told you I wanted to give you ideas. And, I know you still like girls."
Derek slipped it on Sonya, "And, for my first act as your body's overlord, your tongue will still sit naturally in your mouth, it will now grow longer and can extend up to three feet beyond your lips when you stick it out. It can also move essentially in any way you can imagine, without the chance of you swallowing it by mistake."
"I think I know why you gave me this first. And, I'm pretty sure, you have other ideas, too."
"You better believe I do."
"And you didn't want to be changed to half horse just because the cow was uncomfortable, because I know you were enjoying that."
"It's suddenly gotten very difficult to stop and break for food. Sonya, just above your pussy lips, you're going to grow in a horse cock. Three feet long. Erect. Multi-orgasmic. Eight horse balls just below it instead of two, so you can shoot lots and lots of wonderful cum. Lots and lots of stamina. And to return the favor, you now have six arms and six breasts, in pairs, in the same arrangement you gave to me."
The head of her new horse penis was just above her eye level, and with Derek still reclining, she decided to waste no time and just mounted him. One of her hands was behind her, playing with her pussy, and stroking her clit, which Derek never took away in favor of this incredible penis. She used her hands to guide this monster cock into Derek's waiting pussy, which slid in faster than she'd have believed. Mounting this boyfriend mare was an incredible experience. Now, her five free hands and arms were like a lamprey eel, grasping at Derek's equine underside, just trying harder and harder to pull so her new cock could go in further. She started pumping, in and out, faster and faster, cum starting to shoot into Derek, but one ejaculation starting right where the next left off, and she was like a raging animal, provoking more and more for what seemed like ages. One orgasm following another, she was aware of Derek moaning and bucking and finding a rhythm with her, but she simply couldn't stop until, finally, a dozen or so minutes later, her body finally lay flaccid atop Derek, while her cock, still buried to the hilt, was still mostly erect and pulsing and gearing up for more. "I must be slipping. Even after all that, I haven't knocked your socks off."
Derek chuckled in a sweet, high-pitched, breathless tone. "Had enough?"
"My dick still wants more. A lot more. I lost count how many times I got off in there, and I need it more than when I started."
"Alright, if we're going to do this, let's do it right. I don't want to leave you unfulfilled. Sonya, above the human waist, you'll stay exactly the same, and you'll keep your sex organs, but, below the waist, aside from those sex organs, you're now connected above a horse mare's foreshoulders, a centaur." Sonya's body changed, and where she was still hilted in Derek, they were now two centauresses, one mounted on the other.
"I'm glad my pants were already off, that was a much more intense change this time."
"Guess I'm glad I could have the wardrobe malfunction for the two of us." Derek didn't waste any time, wrapped his legs against her equine lower half to hold it, and reached out with all six hands to start playing with her breasts. Almost instantly he was rewarded from a pulse from her shaft, a shudder from her body, and movement from her body trying again to find a good rhythm for more sex.