Liam's immense cock almost fully withdraws from you, only the head remaining inside your dripping folds as his exposed shaft glistens with your juices. As he rams it back in with a wet schlop you gasp as you feel your spine stretch, the tingling sensation intensifying in your spine and ass as pressure begins to build in your tailbone.
Settling into a steady rhythm Liam's hips slam into your ass again and again as you moan helplessly, your erect nipples pressed into the glass of the viewing window. With each thrust he makes you feel your butt cheeks expand slightly as your hips widen, the pressure in your tailbone still growing by the second.
"Uunrggh...." Liam pants, "that's it, you'll need a big rear to safely push out foals" he taunts, running a finger down your fattening ass cheek. The thought alone of being a fat, pregnant mare is enough to send you over the edge, with a scream the pressure in your spine finally bursts like a damn as your tailbone extends with a wet crunch, morphing into an equine tail.
Your eyes roll back in your head as the most intense orgasm of your life hits you, your equine cunt pulsating around Liam's horse cock. Near senseless you barely notice the urine running down your thighs as you wet yourself, your legs shaking with the overwhelming pleasure of your climax.
Overwhelmed by your mind shattering orgasm you are blissfully unaware as your growing ass starts to reshape into haunches, and the first of the fine hairs that will soon coat your body start to sprout from your skin. Unabated the tingling sensation of your transformation spreads further still.