You instinctively know the dog is female, the feeling of air against your fully exposed tip tells you that as well. Sliding the window back open you head outside and decide to let the bitch take the first step. Her head lifts up and turns as you approach, you decide she must've just gotten here a few days ago because she's not acting suspicious of you.
You pull up the metal bar at the fence's base and close yourself in, this is a dumb thing to do with a dog trained to take people down but you feel more than comfortable around her. You let her sniff your hand and you take a look at her collar "Helen, a good name for a german shepherd bitch". You take the chain off her collar and she immediately forces her muzzle into your crotch.
You were already crouched over to unleash her, and her nose finds your dog hair easily through your loose shorts. The feel of her nose, tongue, and whiskers massaging the inside of your crouch leaves you in a dumbfounded ecstasy on your hands and knees while Helen continues about her business. She soon forces your shorts to your knees while a feeling of warmth makes you take off your T-shirt and throw it on top of the dog house. You pant heavily as Helen moves beneath you, forcing your weight on top of her. Helen's head is now beneath your's, her tail wagging quickly, brushing against your fully extended doghood and sack.
Keeping your left arm in place you rise up on your legs, guiding yourself with your right hand, you give into canine lust and move into her inviting cunt. Your headless dick feels odd moving in and out of her but you soon get used to the sensation and with confidence rest your upperbody on her back. You begin to pant in sync and use your previous experience to adjust to her rythms; adjusting your arms so they're straddling her sides you thrust your own cock in even harder trying to bury a bulge that's grown at the base of it.
With a satisfying 'plop' you lock your phalynx in place and continue thrusting none the less. You stand your forearms square on Helen's back to get a better angle and see that your arms are now the forelegs of an adult german shepherd. The cooling sensation from your panting mouth and pull of your happily wagging tail tell you that you've completley transformed into a dog. Your shock is broken by a release of canine seed and your and Helen's howling in unison.
Realization hits you like a ton of bricks and you try in vain to dismount. You don't know anything about dog anatomy and try twisting yourself off. Helen moves and sends you down painfully butt to butt. It dosen't feel so bad right now, fortunately you left the window open so you can crawl back in. Hopefully the power of the dildo would wear off soon and you'd be completely human again, but you don't know if that's even possible now, waiting for your knot to loosen, having probably just sired a litter of police dogs. Katy probably knew what was going on and she'd turn you back eventually, everything seems under control.
Then you hear a car pull up...