The big dog started humping against Luke's crotch, and the teenager quickly sported an erection of his own. Already naked, Luke pushed the Great Dane off, but then knelt on the floor. Dropping to his hands and knees, he looked back at the canine, who had perked his ears. "You want me, go ahead." The dog needed no further prompting, though shifting behind and over Luke was awkward. The teen felt a hard prod against his anus, and then winced as the slick dog cock pushed in with surprising force. The male Great Dane was big, and Luke gasped and clenched his hands as he was mounted. He felt hot, and very odd. His guts were shifting about, and the throbbing erection brought about by the dog's attention started to ease.
Luke dropped his head to look between his legs, and saw that his penis was not only losing the erection, it was getting smaller! He did not understand until the dog shifted, pulling out before he knotted and thrust back in a new opening just below Luke's anus. The teen's eyes widened as new sensations washed over him, and he watched as his penis shrank down to the nub of a Great Dane bitch. The magic of the costumes must still be in force! There was no pulling free, for the male had swollen out and was knotted inside him - or was the teen a her now?
They remained tied for a long time, the male finally pulling free with a yank that had both of them cry out. Luke felt his rump and discovered the beginnings of a tail, and his buttocks felt fully canine. The male returned to lick at Luke's sensitive cunt, which sent renewed lust through the teenager's mind. He presented himself to the male again and was not disappointed. This time the pleasure was much more intense for Luke, the need and instincts of a female Great Dane filling her mind. Luke was vaguely aware that her body was altering quickly now. and felt as well as saw her face push out into a muzzle.
When the male finally slid off her, Luke shook herself and twisted around. It was as she expected - she was fully a Great Dane bitch. So why did she still want another round with the male? As he sniffed her, Luke saw that the male's eyes had changed - they were dog's eyes, with dog's brain behind them. It wasn't the kid anymore, it was a male Great Dane! And Luke realized she wanted to join him all the way as an animal. Was that the costume's influence? Did it matter? She presented herself again, and the male responded happily.
Mating was smoother this time, a matched set of animals. And as the big male thrust and humped, Luke's mind began to simplify. Pleasure and need, raw instincts took control of her actions, and memories of being human began to fade. She was still Luke, but soon she would Luke who had been born a dog, not human.