As your knot locks you inside Duchess, something inside you seems to click. You have always been like this, you always have been naked and walk on all fours since the day you were born as a litter with 3 brothers and sisters. It feels too right to have your cock inside duchess for it to have ever been anything else. You pant contentedly as you look at your master.
He smiles at you and says, "Good Boy! We should get some good pups out of that!"
You can't understand the words but they sound happy which makes you feel that he is pleased with you. This though makes you very happy and you gaze lovingly up at him. As you look at him you notice that the fur that covers his body seems to be thinning out. Even as you watch, more fur disappears leaving only smooth flesh behind. As this happens, his tail also disappears as his spine shortens and the skin tightens over his ass. What was left of his muzzle recedes into his face becoming the face of a human in his mid 20's. he doesn't seem to notice any of this as he looks down at you and your now impregnated bitch.
As his changes finish, you feel your self begin to change as well. Fur grows all over your body, starting at your cock, then sweeping out ward across your back and chest before covering your legs and arms and head. Your short muzzle extends into a full canine snout and your tongue still hangs out of it, moving up and down as you pant. You feel a tail forcing itself out of your back, stretching out until it's over a foot long. As soon as it finishes its growth, your tail immediately begins to sway from side to side. The changes complete and you are now completely a dog. Your knot deflates enough for you to pull out of Duchess as the last vague memories of humanity fade forever. You lick your cock clean as it returns to its sheath while Duchess cleans herself up.
Your master looks proudly at you before giving you both a meaty treat. Your tail wags with happiness as the flavor fills your mouth, as you fully accept you are nothing more then a pet to serve your master. You scratch at an itch behind you ear with your hind paw, content and happy with your new life.