You cry out as you feel your rear begin to stretch back, mere moments later you bellow as the pain in your hips intensifies and a pair of equine legs burst free, taking your weight as your original pair are dragged backwards with the rest of your lower body. You moan as your stretching lower torso bloats and expands, your still erect cock shifting downwards to hang beneath your increasingly large and hairy lower body.
"Centaurs!" Your girlfriends cries out as she realises what you're becoming, "We're turning into cen...ahhhhh!" she screams mid sentence as she falls forward onto all fours, her feet blackening as they harden into hooves, her still growing ass becoming more and more like the haunches of a mare. You akwardly rush to help her on your new unfamilar legs, your lower body now almost fully a horse's.
As your girlfriend sits back up she squeals and clutches at her chest as her boobs start to rapidly expand. Before you even know what you're doing you grab the neckline of her shirt and with a hard tug force her arms up as you rip it clean off, "Babe!" she yells, glaring at you as you akwardly drop her shirt, embarassed at what you just did you apologize, though your eyes stay glued to her chest, new instincts beginning to take hold as your mind begins to subtly shift.
Staring in amazement you watch as your girlfriend's boobs visibly pulsate as they expand, her errect nipples joining in as they rapidly swell and lengthen into four inch long teats. Inexorably her increasingly massive tits begins to slide down her chest, your balls pulling tight at the sight as a tail shoots from the end of your equine spine as you watch her moan and buck her hips.
As your girlfriend orgasms, overwhelmed by the sensation of her migrating breasts, she falls forward onto all fours again as her huge breasts force her legs apart, settling into their new position between them as they subtly change shape, darkening and leaving her with a pair of mare's teats.
Coming out of her orgasm your girlfriend's eyes immediately fixate beneath your now fully centaur body, you ask her if she's ok but her only response is to lick her lips as she starts to crawl beneath you, she knows what she wants. As her lips wrap around the head of you cock a feral shiver of excitement runs down your new, longer spine as the intense breeding instincts of a male centaur fully take hold within you.