Cal was enjoying the night air when eh spotted the buck. To have one of the impressive animals so close was surprising, and this one seemed more curious than fearful when it saw him. Extending his hand out, the boy took a few cautious steps towards the beast as he spoke soothing nonsense. He became aware of flickering lights around the stag's head, and then around his own, but they appeared to be large fireflies. There was something odd about the variety of colors, but he only had eyes for the stag.
The animal flicked its ears and took a few steps closer, snuffling in Cal's direction. At the same time, the large bugs flitted closer to the boy, landing on his shoulders and head, then crawling over his body. There was no danger from fireflies, and Cal did not want to make any sudden movements to shoo them off for fear of scaring the buck. He grinned when he realized the stag was showing a different kind of reaction - his impressive member was dropping.
Another few steps and the big male was snuffling Cal's face and lipping at his ears. Moving slowly, the boy reached out and gently rubbed the animal's neck. They shifted closer, until the boy was rubbing the soft fur of the stag's neck and chest while the animal lipped and licked the side of Cal's head, matting his hair with spittle. After a furtive glance to make sure no one was watching, Cal slid one hand further down, wondering if the stag would allow intimate contact. He felt the warm, furry sheath and then the rigid organ itself, slick and hard, much like a dog's. The stag paused, causing Cal to freeze, but then the animal renewed its slobbering over the back of Cal's head.
Disappointingly, his fingers seemed to go numb and his own erection was lost even though the stag was obviously very interested in Cal. Puzzled, the boy lifted his hand to see what was wrong and blinked in confusion at a nicely formed cloven hoof. One of the glowing bugs landed on it as he stared, and dark fur spread up his arm as the bones shifted under the skin. He felt the warm breath of the stag against his back, and realized his shirt was missing. Looking down, he saw the last remnants of his pants crumbling off furry legs. One of the glowing bugs was crawling over his crotch, and as he watched a swelling grow. But not his penis - this was rounded and had protrusions - teats! He had an udder?
Cal became aware of many things all at once - the buck's scent strong in his nostrils, a sense of growing need in his mind and dampness from a new opening in his rump, and a loss of balance that suddenly had him pitching forward. He caught himself on forelegs instead of arms, nearly fully transformed into a doe. He... she could smell and see the male's erection and gave the swollen organ a lick at the same time the buck licked her rump in a sensitive spot.
The former boy flicked his tail and urinated instinctively, then braced as the male shifted position and then covered her in a smooth motion. She bleated as the male pushed in, mating with her quickly to fill her womb with seed, then sliding off. They sniffed each other, then the male headed off into the woods. Cal looked back at the camp, already confused over why she had come here. The stink of human was everywhere, but she also had a strong sense of accomplishment, primal purpose fulfilled. She would carry the male's offspring, give birth, wean, and repeat the cycle. She followed her male into the woods, fully a doe.