This was written because, inexplicably, there didn't seem to be one already.
The Kingdom of the Sun has been waiting for its lost princess, a girl with golden hair, to return for over a decade. In this story, when a girl with golden hair named for their Princess shows up, people notice. And also, other stuff happens.
AU. Best read in 3/4 format. Flynn worked out that Rapunzel was the Rapunzel after her little trick with his hand.
Life, Untangled
Flynn, being the professional thief that he was, noticed it long before Rapunzel.
The sideways looks. The double-takes. The quiet whispering.
The stares, at the girl with impossibly long golden hair and green eyes glowing with excitement.
The girl who could be a grown-up version of the infant princess so lovingly detailed in mosaics around the kingdom.
And the girl, he realised belatedly, he'd incautiously been addressing by name all this time. Knowing his luck, it wouldn't be long before they were both arrested for trying to run a scam. Especially once they twigged who he was.
Actually, he was a little insulted they hadn't already. Surely he was more famous than any long lost princess?
Oh well. No accounting for taste, he supposed.
Annnnnd that nice couple over there was now talking to a pair of guards and gesturing sharply towards Rapunzel.
"Time to go!" He said cheerfully, breaking into the ignorant princess' lantern painting by the expedient method of gripping under he arms and lifting. "Lots more to see before the sun goes down, come on now, no whining kids it gives you wrinkles!" He half-dragged her away to a chorus of plaintive wails. Seriously, they needed some strange girl to paint lanterns with them? Kids these days were so selfish. He could see the guards' reflections in a nearby shopfront window. Were they? Yes. Yes, they were totally coming this way. Oh man!
In hindsight, his hauling a pretty girl away to loud complaints wasn't the smartest idea in the world, but he was still pretty sure he could have gotten them away and out of sight if it weren't for the horseless cart that suddenly came thundering down the wide main street.
People screamed and threw themselves to the side as the heavily-loaded four-wheel cart ran out of control at top speed down the hill and toward the centre of the square - right towards the children playing.
Rapunzel screamed in horror, lunging against his grip which was only tighter in reaction to his own shock. The guards and several adults ran forward but the eldest of the children - eldest sibling, by the look of them all - was faster. She grabbed her brothers and sisters and shoved them away, snatching up the smallest as she made to run herself.
At the very last instant, as she registered that she couldn't possibly get away in time, she threw her tiny sister into the outstretched arms of a guard.
And was crushed by the cart.
"NO!" Rapunzel screamed again, a cry taken up by all around. The cart kept going, stopping only when it hit the bookstore at the other end of the square. In its wake, it left the broken body of the young girl it had run down. Only her legs were left untouched. Both her upper arms were twisted, half her chest was caved in and blood filled the crevices of the cobblestone road from the place where her head had been slammed into them. Stark white and silent, she looked dead.
A woman screamed, louder and more frenzied than Rapunzel by far. Clutching the rest of her children, the girl's mother sunk to her knees and wailed. A man, probably the girl's father, trembled as his hands fluttered over his broken child. Both wept unreservedly, their children quickly following.
A guard checked the girl's pulse, shook his head, stepped back. The other guard removed his helmet and bowed his head.
All around the square, people cast their eyes down in respectful, horrified sadness.
"NO!" Rapunzel snapped, slamming the heel of her foot into his arch. He bucked at the sudden pain, releasing her reflexively long enough for her to get away. She ran to the man and his child, heavy braid swinging behind her even as one hand tore savagely at the woven flowers and vine ties, pulling viciously until one long rope was tugged free. She dropped down to the side of the child, dodging a guard's startled attempt to stop her, and threw her freed hair over the girl.
"What do you think you're doing?!" The father shouted, voice so wrecked with tears it was barely understandable. "Leave her alone!"
"I can save her!" Rapunzel insisted, hard and certain and forceful in a way he'd never seen before. The father just shook his head, pushing her away, yanking her hair off his daughter - hair that was staining blood red in some places.
"No… no…"
"Let me try!" Rapunzel gasped, shoving back, fighting to do what she'd been so conditioned not to do outside her tower that the two of them had almost drowned once. "Even if it doesn't work, at least we'll have tried!"
The father's expression - and will - broke. He stopped pushing her away, instead just curling over his daughter to weep. Rapunzel ignored him, re-wrapping her hair around and under the girl as fast as she could.
Like everyone else, Flynn held his breath. There was a big difference between a sliced hand and… and this.
But he hoped.
"You can do this." He whispered. "You can do this. Rapunzel. Rapunzel."
Finished with her hair, the unknowing princess smoothed a hand down the girl's side, closed her eyes… and sang.
"Flower, gleam and glow
Let your power shine"
Strangled gasps went up all around the crowded square as, in the growing gloom of evening, Rapunzel's hair began to glow.
"Make the clock reverse
Bring back what once was mine"
Light poured from the roots of her hair, down and through, following the weaving pattern of her braid and out again to wrap around the broken body of the girl. Tiny specks of light floated up and away, like minuscule replicas of the lanterns that had been launched every year to call her home.
"Heal what has been hurt
Change the Fates' design"
Wreathed in the mystical power of Rapunzel's hair, in the power of the sun, the girl gasped loudly as life returned to her. Wide brown eyes, gleaming gold in reflected light, stared at her rescuer in astonished awe. She seemed to not even feel her arms return to their normal shape, nor her chest re-expand.
"Save what has been lost
Bring back what once was mine"
Every man, woman and child in the square held their breath. They all knew that they were witnessing a miracle. The quicker thinkers among them knew also that at long last, they had found their princess. That she had found them.
"What once was mine…"
The song - the spell? - ended along with the light show. Rapunzel opened her eyes and smiled in plain relief to see the girl staring back at her. Gently, she unwound her hair from her body, the girl herself helping as she sat up. Once she was clear, the father clutched his daughter close.
"Thank you." He choked out, loud in the silence.
"Thank you… Your Highness."
"Aw, crud." Flynn said right after, before a rising whoop of noise swallowed it whole. The citizens rejoiced, shouting and laughing and singing in joy. Some ran to spread the news. One guard began to hit a bell loudly, to summon others, as his partner moved carefully closer to Rapunzel - and bowed deeply as she turned to him.
She stared, absolutely uncomprehending. Lost green eyes sought him out in the crowd, but Flynn could only shrug helplessly, stomach churning in loss.
Whether she knew it or not, she was home now. For real. And no way would the kingdom ever let her be taken again.
And no way was a thief and a liar like him good enough for any princess, let alone Rapunzel.
He tried to smile. It was for the best, really it was. And hey, her parents would probably be grateful enough to buy him a palace of his own, on an island of his own. Right? Right.
Everything he'd ever wanted. Right?
…Right.
Fin
There didn't seem to be a set moment in the movie when Flynn discovers who Rapunzel is, but considering that he knew what the lanterns were about right at the start and the mystical events surrounding the princess' birth would probably be common knowledge (who doesn't talk up magic flowers that everyone in the kingdom searched for and found?) and that the girl with green eyes and golden magical healing hair shared the same name as the princess (according to the Disney court of Princesses)… well, he's an idiot but he's not stupid.