"Phew," huffed Henry. "Is it getting hot in here?"
Linda answered by turning up the air conditioning.
"I hope the damn thing isn't giving out," grumbled Debra.
"Language, dear," responded her mother automatically, prompting the teenager to roll her eyes.
However, even the increased blast of cold air did nothing to assuage the heat sweeping over them. The whole family began to squirm and pant as the supernatural warmth melded with their flesh and blood, clothes dampening with sweat. it was not an unpleasant feeling, exactly; more like being in a sauna.
"Wh-what the hell," muttered Henry, the drafts from the air conditioner cooling the sweat across his shirt front even as the heat continued to pour from within. Settling in his chest and his groin, it pulsed with his heart, his blood echoing in his ears as his cock began to swell. But it was different, somehow, as if he weren't just coming erect, but the whole organ were increasing in size and virility, and his ballsack seemed to be filling out his briefs to match...
"Dad, I feel weird," complained Rick, shifting in his seat.
"You are weird," rejoined Sam, but without much enthusiasm.
"Ehhh." Debra eyed her brothers' expanding crotches dubiously. It was pretty awkward, seeing her kid brothers come erect in front of her. but soon she had issues of her own. Her pussy would no longer be ignored, and she drew her breath in through her teeth as she felt it... move? She grit her jaws, telling herself that was ridiculous, then gasped as she felt something -- two somethings, firm and round -- push out between her vagina and her clit and jostle around in her panties. And her clitoris -- as it throbbed, and throbbed, it seemed to be getting bigger, harder, and even more god-damned sensitive.
"D-Dad," she gasped. "I think we might be... nnn... sick or -- or something..."
"Yeah," he replied hazily. Red was starting to obscure his eyes as he felt his entire body begin to expand, hot masculinity surging through him as his muscles bulked and pressed against his clothes. The white fur overtaking his dark body hair was not helping either.
He had just enough sense to pull to the side of the road before he succumbed to the feelings completely, the groans of his wife as she fondled her growing tits filling his ears. He wanted to ask what was happening, but he could only vocalise in lusty pants as his jaws stretched out, his tongue writhing uselessly against larger, flatter teeth.
"Oh!" Linda had torn upon her blouse, and snapped the band of her far too tight bra, displaying snowy white bosoms for the world to see -- the world right now being her husband, as nobody else was on the highway. But he made a more than sufficient audience, his darkening eyes ogling her with an appreciation she had not felt since... maybe ever. Both spouses felt their 20th century inhibitions deserting them, forced to retreat before primal equine instincts.
And God did it feel good. With a grunt Henry flexed his body, his shirt and jeans ripping down their seams to release powerful stallion flesh, his belt tearing away from the buckle as an equally powerful stallion cock pushed forth. He grabbed it and began pumping, the shaft already lubed by precum and sweat.
Similar changes were overtaking their children in the back seat, In fact, Debra was feeling the full duality of her parents' changes. As her pussy puffed and reformed to a muscular mare's cleft, and her breasts and hips filled out from lanky teen to fertile breeder, so too did her ballsack fill with potent seed and her former clitoris grow to crown it as an eight-inch length of horse meat.
And so thick her fingers would not have been able to reach around it if they had not also grown; long, slender but strong. Black paint flaked off her nails as they turned to naturally burnished jet, expanding to cover the entire back of her fingertips, the insides left in smooth, sensitive dark leathery skin that felt heavenly rubbing against her veiny shaft.
She was vaguely aware that she ought to be mortified to masturbate her new maleness in front of her kid brothers, but hell they were doing it too -- just as she could tell that her parents were rubbing their own pleasure out.
"So clossse," she hissed between her new equine teeth, eyes squeezed to slits as her free hand squeezed at her own ass. "Sooo -- ah... ah...!" She felt -- as if something were restraining her, but that it could only hold her back so long. Fierce and exultant, a new fire burned in her prematurely jaded blood, and it would soon be free -- "Oh God!"
Hers was the first of the chorus of satisfied cries echoing in the enclosed cabin as its five passengers found release. And with it, an ancient power sealed its gift to them and the modern, wonder-starved world.
Light flared in their vision -- not just from a mind-melting orgasm, but because brilliant stars had burst forth from their foreheads, incandescent pools of magical energy from which emerged long, translucent, spiralled horns. From nearly two feet on Henry to immature four-inchers on the twins, this marked the culmination and completion of the family's transformation into unicorns.
Henry undid his seat-belt, which had strained to the limit but held when everything else gave way. Huffing in deep breaths, he looked over himself in wonder. He was a white stallion, his brown hair having deepened to glossy black mane, matched by a thick horse tail. His horn, seen in the rearview mirror -- On my God, I'm a unicorn! -- was a sort of golden amber, motes of light swirling inside it.
Looking again at his wife, he saw that she was pure white, with a silvery horn. And, again utilising the mirror, he noted both the gender change in his daughter and her appearance -- light pink body, cherry-red mane with black streaks, and a horn dark red at the edges but lit within by passionate crimson fire. To either side of her sat young black stallions.
"...Wow," he managed. The word seemed to snap everyone else back to reality. For the moment, the rush of freedom and pleasure they had experienced in their change faded to the backs of their minds.
"Is everyone all right?" asked Linda anxiously, studying her children in the mirror as well.
"Yup. Just... turned into unicorns," Debra tried to snark, but she could not hide the awe in her voice.
"Whadda we do, Dad?" asked Rick, bouncing in his seat.
"Well..." Henry glanced down at the steering wheel and dash. Given the copious amount of unicorn seed he'd felt pumping from him, he expected the controls to be covered in a layer of white, and probably ruined. But to his surprise, the dashboard shone as if newly cleaned, and he swore the leather padding on the wheel looked more plush.
In fact the entire vehicle had shifted in subtle but noticeable ways. For one thing, it was somewhat bigger, affording the passengers more elbow room. And the seats now had openings in the back to allow their tails to swish freely. Outside, the sun-dulled red had deepened into a rich burgundy, that gleamed as if newly brought onto the showroom floor. On the hood, a new silver unicorn ornament reared above the sparkling chrome fender.
"Let's... stop at the next town," he decided. "We need time to rest and... think." He stole a glance at his much-curvier wife, snuffing the scent of fertile heat pouring form between her legs. There were more important things to be done than think.
"Will it be safe?" asked Linda. "I mean, we're..." She gestured at herself.
"Yeah," piped up Sam. "What if the government captures us and experiments on us?"
Henry shook his head. "I'll protect you," he declared confidently. "Can't you feel it? How... how alive we are?" He stretched in his seat, flexing as well as he could. "And I feel..."
"Magic," finished Debra, nodding. "Yeah. I don't think we have to worry about some little desert town, Mom. We're all fucking unicorns -- I bet even the squirts could take on a police squad or two."
"I... I suppose you're right, dear," agreed Linda, hesitant at first, but feeling more confident as the words of her stallion made her realise -- she did feel alive -- youthful, gorgeous, and magical. She smiled, an unwonted mischief creeping into her thoughts as she reached up to stroke her bare breasts again. "No -- I'm sure we can take care of anyone who tries to cause us problems."
This time it did not occur to her to reprimand her daughter's language -- like sexual taboos, that concern had floated away, frail and nebulous as a cloud before the desert wind. She settled back into her cozy chair, still fondling herself.
Henry carefully set a cloven hoof onto the gas, not sure how it would work, but to his delight found the pedal had molded to fit him. Gunning it, he pulled back onto the empty highway, pushing past the speed limit. It wasn't the same as running freely on his own, but the car was still faster and it was still over 100 degrees outside, so this would have to do for now.
Apart from filling his wife with some new foals, his plans for the future were unclear. But one thing was certain -- their sedate vacation was over. For now it was enough to count the signs until they got to the next stop and could "relax" and enjoy their new forms properly.