"This one will be excellent for our procreation purposes", the centaur exclaims. Taking a look around the room as far as the table straps permit you to, reveal their need for females: the centaur population consists almost entirely of men. But not for long...
The centaur vet opens a drawer, takes out a vial and says:"Ah, this centaur blood specimen will be perfect for the job." He fills a syringe with it, which you gaze at in fear, puts it in your arm and squeezes. You can feel the surprisingly warm centaur blood rush through your arteries, already causing your heart to beat faster. Fine white hair grows on your skin beneath the waist, quickly becoming very warm as the hair thickens. In an incredibly painful twist, your knees reverse with a loud pop, causing you to clumsily twitch your elongated legs in pain. Your feet melt into one solid mass of flesh, quickly getting darker and eventually turning a shiny black, leaving nothing of your feet but two shining black hooves.
You watch two masses of flesh grow out of your stomach, which you seem to be able to move without a problem as the bones and muscles in them are directly wired to your nervous system. They grow as long as your first pair of legs, covered in the same white fur and ending in two shining black hooves. The transformation so far has left you almost completely skinny to the bone, but suddenly you can see your belly gain pounds, leveling your belly with your chest and leaving you with an admirably fleshy front.
Your crotch has almost completely realigned itself and you're glad you're unable to see your privates because they feel huge all of the sudden, followed by your butt inflating to equine proportions with the rectum between it following suit, lifting you off the table a bit as your body now practically rests upon your butt. Above your butt, you can feel your spine expand, a rope of flesh growing downward, quickly covered in beautiful blond tail hairs. Concluding the transformation, your ears gain the same white hair all over, rearranging into pointy controllable ears which you instinctively twitch back and forth a few times. Your ears are now almost on top of your head, sticking out between your hair.
The vet finally releases the straps on the table and the other centaurs help you get up, landing on the floor with a light clicking sound of your new hooves. Too exhausted by the transformation to resist, the centaurs lead you out of the building...