Katie kept licking Tiffany's cock, as more and more cum dripped out. Her large tongue savored the taste, and she released a soft moo. Tiffany was barely aware of her friend's presence. Her mind was focused on her bubbling skin.
Some of her animal features slowly melted away, her muzzle retreating back into a human shape, and her tail shrinking into nothingness. Fur receded, and smooth pink skin was left in its wake. But Tiffany's endowments remained. As she sat up, her breasts hung painfully from her. They were so heavy, each filled with milky cum. Her nipples dripped liquid, and were still massive and black.
"Oh god!" Tiffany said, as more changes started to take hold. Her hands and feet remained hooves. Tiffany brings them up to caress her breasts, and then down to push Katie's muzzle further onto her shaft. Katie engulfs over half of it, and Tiffany cums.
When Katie pulls off, she notices Tiffany's rear appears larger. Katie squeezes Tiffany's ass with her own hoof, and is rewarded with a deep moan from her friend. Tiffany bucks her hips forward as her body grows again.
"Fuck!" she screams as her ass pushes out. She accidentally shits as her ass and internal organs are rearranged to make room for her new body. New legs grow in and start to thicken until they're identical to her front pair. They too end in crude bovine hooves. Tiffany slams down onto the floor, and smashing what remains of the vials.
Katie is watching with rampant fascination as her friend is transformed into some sort of weird "taur". Between her front legs is her massive bull cock and scrotum. But between the back legs, a cleft has formed under her ass hole. The cleft deepens, and soon starts to drip liquid. A new musty scent is released into the air by Tiffany's new vagina.
Katie, overwhelmed by her bovine instincts, can't wait any longer. She turns around, presenting her twat to Tiffany, who had been caught up fondling herself. As soon as she catches a glimpse of Katie's pulsing lips, Tiffany stops playing with her chest, and slides her penis inside. Katie moos in pleasure, while Tiffany starts to pump in and out.
As she does, her balls descend further. They reach her knees, when Tiffany feels something of. She glances down while she fucks Katie and gasps when she realizes there are two lumps above her testicles. The swell, and soon she had another set of balls directly above her original ones.
All four pull up, and Tiffany has the most intense orgasm yet. It's so strong, Katie's cunt actually splits from the sheer force. Blood spills out of her cow pussy, but the pain is remedied by the pleasure she feels.
Both girls (if you can still call them girls) pass out on the floor. Katie's breeding cow body and mind are set, and permanent. Beside her, Tiffany the herm-taur is still leaking fluid and cum all over the floor. Her breast milk, cum, and lady fluids make a creamy pool underneath the two. Katie's tongue laps at the mixture, and she moos in her sleep.