The blue latex quickly filled Susan's pussy and came flooding out staining both your and Susan's crotches. While you and Susan were distracted by your orgasms, the latex then began to coat both of your legs, fusing them together as it crept downwards. Wherever the latex covered began to change, your fused hips shifted into a quadrupedal orientation and a latex horse tail grew out behind you. When the latex reached both your feet, it fused them into one mass that hardened into hooves.
It was at this point that you had recovered from your orgasm and attempted to pull out of her, only to find that you couldn't. You then look down at your fused horse legs and realize what had happened. You try to get Susan's attention, but she seems to be in some kind of trance, leaving you helpless as the latex begins to continue up her torso. As the latex flows up Susan's torso, you feel a sensation in your groin as a two foot latex horsecock grows out below Susan, appearing to be a larger version of the dildo.
The latex has now taken over all of your girlfriend's torso, her breasts consumed by a barrelling equine chest. The latex then flowed down her arms, transforming them into cloven forelegs. As the latex begins to cover Susan's head, your torso starts sliding forward. You reach the front of Susan's body just in time to see her face completely covered in latex, becoming indistinguishable from the font of a horse. Once your changes have settled the latex dissolves away into nothing, revealing blueish-grey fur covering your entire lower half. Looking behind yourself you see your tail's hair is the same shade of brown that Susan's hair was and, when you look at yourself in a mirror you see a horse-pussy sitting alongside your ass-hole beneath the tail, as well as your large equine cock and balls.
You begin to panic, wondering what has become of your girlfriend, calling her name as you look for any sign that she isn't effectively dead on your body. Just as your about to break down in tears, concluding that she is as good as gone, you hear her voice.
“Don't worry Jessie, I'm not gone”
Your eyes go wide as you realize where her voice is coming from.