So, I find out she has a potion. Not for drinking, unless I want to fix the changes in place and make them permanent. But, I get undressed as she tells me, and she pours a little into her hands...and starts rubbing it into my dick. And my dick grows longer, very quickly, staying hard and getting longer all the time, until it's about as long as I'd expect on a horse...or a centaur. At that point, she stops and tells me we could add more to change me into a centaur, and we can do it any time we like. For now, the potion will keep me hard and long-as-a-horse for about three or four hours, and she wants me to take full advantage. At that, she stops fondling my enhanced appendage, lays back on our bed, and tells me to mount her.