You tense instinctively, ironically like the proverbial dog who knows he's about to be hit. But instead of a blow comes the vaguely familiar sensation of the man's transformational magic.
Still on all fours, you have a pretty good view of yourself as your body starts to shrink. You can see the hair on your arms and chest rapidly vanish, the bush surrounding your insistent erection receding, your legs growing smooth. Your chest and shoulders contract, growing less broad, and your body loses much muscle mass, giving you a lanky sort of look.
You're starting to look a lot like a young teenager, but some distant part of your mind notices and approves of the fact that your skinny limbs and narrow body look much more appropriate for a dog-slave like yourself than your previously manly self, giving you a canine sort of leanness.
Though your body has passed somewhere into the early part of puberty, your cock has stayed the same size, your thick seven incher and heavy balls dangling lewdly between your skinny thighs.
"Don't you just look sweet and innocent, boy? Hah! Except for this! Sometimes I like a little meat for myself."
The biker reach under you and fondles your man-size equipment, triggering an instinctive submission response in the dog part of your brain. As he runs a meaty finger along your exposed boy hole, you feel the impulse to raise your tail, your hips wiggling of their own accord.
"That's right, boy," he says. "I ain't quite done. Wouldn't do for you to be quite so pretty."
And now you can feel the tail you wanted to lift so badly, just a nub now, but you twitch it upwards anyway as it grows out, bushy fur sprouting along it like when you were a full dog before. You can feel the light tickle of the fur spread down to your heavy balls, covering it over, and then an odd tightness around your throbbing cock. You look down and see a short canine sheath around the base of it, squeezing snugly around the engorged shaft. Your cock seems even longer than before, reaching to your bellybutton, its length extended to match the extra covering of the sheath.
You can feel your hands and feet changing now, and you watch as your hands curl up on their own accord, fingers bent to resemble a dogs paws, and difficult for you to straighten then. They seem to swell, and darken slightly, looking oversized on your teen boy frame, but you find they're easier to rest on now. Your feet also seem to grow and swell, especially around the toes, which darken slightly also. You can see your toenails thicken and darken, resembling blunted claws.
The rest of the changes are less easy to see, but you can feel it as your ears stretch out and darken, not into dog ears, but longer and thinner than human proportion, darker like your fingers and does. Your face aches for a moment as it pushes into a very short muzzle, your nose flattening out and turning dark and damp. You drop your jaw, allowing a changed tongue to flop out over darkened lips, thinner and longer. You pant slightly, cooling like a dog would but still sweating like a man- or boy?
The man chuckles.
"I'm eager to try out that muzzle and tongue of yours, pup. Not too many of the guys in the slammer could take me all in, but I bet you can now!"