Maddi stated her bet in such a way that it really only benefitted her, however as she now has control over tour body, there is little you could do to hinder her. And, there was no way either Liz or Amanda could out run you, even if you were only a human-taur, 4 legs were betrer than 2, so you both won with ease, winning the bet.
You could feel the changes happening as your back leg's knees reversed and now bent differently, becoming more horse-like. Your dick grew longer and thicker, gaining a sheath in the process as your gland flared and flattened out into a magnificent horsecock. Your balls swelled too, becomming the size of baseballs. Your asshole became donut shaped and now stuck up and out of your ass a bit. Next, you feel a cavity open up inside of you, just below your stomach, as well as a new opening connecting to it located just underneath your asshole. The cool breeze tickles your new mare pussy as you notice that Maddi's human pussy that is between her original legs, which also is now essentially your mouth, is getting fairly wet as her arousal grows and grows.
Then your body is forced upwards a few inches as your feet now are standing on just their toes. Maddi tries to flatten your feet again, but quickly understands that it no longer is possible. You are going to be forced to walk on either your toe tips, or maybe in 2 tall pairs of heels. Then, a lightly golden horse tail shoots out above your asshole and hang neatly behind you. The color matches Maddi's hair which she ironically, she had styled into a ponytail before she left her house today. Lastly both your own and Maddi's breasts swelled up to EE cups, and yours started lactating as your breast's nipples grew out about 3 inches long, becoming closer to teats. Unlike a normal horse's though, your breasts stayed where they were and didn't relocate more towards your hind legs. However, they were just in the right spot where your erection could slip between them and be held in place by the pressure. Of course Maddi could always force your cock out from inbetween them, but it felt good, so she left it for now as a small amount of precum was slowly starting to coat your lower breasts.
It was just now that Amanda and Liz had caught back up. Neither of them were really trying too hard this time, as they were just there to lose intentionally anyways. And having walked almost the entire way, it just took them a bit to catch back up. Now that they were here though, Maddi just couldn't help herself and began to flaunt her body in front of them. Striking various poses, some of which were even a lot more lewd than usual, specifically like when she made your body sit and spread her own legs wide apart to present her massive erection and your own dripping mouth-pussy, which she was teasing the clitoris of with one of her fingers, causing you to experience a lot of concentrated pleasure.
Amanda and Liz were enjoying the show too. Both girls had stripped naked and were acting as if they were part of Maddi's harem or something. Liz was pressing herself up against your horsecock with her mouth and tongue going hard on your gland, while Amanda was suckling from your right teat as she used her feet to gently massage your balls and her hands to play with your mouth-pussy, to which you found that you loved feeling very much. It was a good thing the area you all were in was practically deserted, or this kind of thing would not have been able to take place.
Maddi while enjoying the tongue work of Liz and the feeling of feeding Amanda her milk, began to address her form. "Well, I think this is as much of a centaur that I want to be. It is the perfect balance between human and horse. But, now that I am set, lets make a new bet for you girls to enjoy.
Both liz and Amanda hoped off of your's and Maddi's body and put their clothing back on, save for their tops, which they partially put on, but left their boobs fully exposed. "Ok, so lets see which one of you can make me cum fastest." Began Maddi. "I will give each of you a fair shot, and you go until I cum, or we reach 45 minutes. You may use any method you can think of as well."
"And what are the stakes?" Asked Liz.