The boy's tennis shorts were beginning to tent quite dramatically as he neared the donkey pair. He was growing hot and sweaty and didn't notice at first, but the closer he got, the harder he got, and his sudden and unexplained arousal got the better of him.
"Oh!" he exclaimed, dropping the stick, grasping the front of his tented shorts. It was like fireworks shot off behind his eyes as the pleasure hit him. Immediately he began stripping off the shorts, his stylish boxers quickly joining them in a pool of clothing draped over his expensive cross trainers.
He took his hard 6 inches into one flushed, sweaty hand and began jerking furiously, unable to stop himself. The still-aroused donkeys turned to watch as he pistoned his fist with an urgency only a teenager can manage.
He grunted and panted and groaned, all inhibition suddenly stripped from him, his knees spread wide as he thrust into the air.
Suddenly, with a shout, he shot his load! The first shot went long and hard, and splattered on the hides of the equine spectators.
And something even stranger happened...
With each shot of cum, the boy changed a little, growing smaller and slimmer. The fine bush of blond hair around his dick cleared away. His cheeks got rounder and fuller, his eyes bigger and brighter. His skin smoothed, and the tight sinewy muscles shrank. His big hands and feet got smaller and smaller, until the now much younger boy could barely hold on to his slippery, hard, and only slightly smaller cock.
Spent, the formerly preppy and athletic teen began to wander off into the field. He left his clothes, now far too big to fit, in a pile by the fence. Now reduced to an adorably tow-headed, cherubicly innocent young boy, his memory wiped clean except for an insatiable desire for sex and an incurable horniness to match, he'd never be the same again.
The two transformed former humans laughed in donkey voices at the spectacle, and wondered what just happened.
And the former teen's cum stuck to their fur was starting to glow...