This takes place during the fight with Zabuza and Haku, in which Naruto is a girl, and this time her name is still Naruto. Just Naru, for short. This is a response to Evil Genus's challenge. Anyways, onward!
In the Ice dome, a sinister chakra was spiking. In one of the mirrors, Haku could only watch as Naruto laid Sasuke on the ground and slowly stood back up. The senbon that had hit him were falling out and the wounds were sealing shut. Suddenly the chakra became hot and oppressive, the pressure breaking some of the ice mirrors and causing Haku to jump to the mirrors farthest from the boy. Naruto turned to look at Haku with red slitted eyes and let out a wordless scream of rage that was so loud, Kakashi could hear it from his battle with Zabuza. They paused their battle the moment they sensed the Kyuubi's chakra, and the color drained from Kakashi's face as he heard Naruto's scream.
'Oh no,' he thought as the dome was pierced by a column of fiery chakra. He ran to the dome, and was surprised to see Zabuza running with him.
"Kakashi! What is releasing that chakra?" shouted Zabuza as they reached the edge of the dome, in time to see Naruto in a crouch above Sasuke, and ice shards everywhere. Haku was just hopping out of one of the only mirrors that weren't destroyed.
"It can't be," whispered Zabuza. Naruto was snarling incoherently, but something was off about him. His eyes were red and slitted, and his whisker marks were more pronounced, but it was something else. Kakashi couldn't place it, but there was something different. But before he could say anything, Naruto lunged towards Haku, and punched him in the face. He raised his other fist to punch him again, but before it landed, he stopped a centimeter away from the breaking mask. Half of the mask broke off, revealing Haku's face.
"You're that boy. The one from the woods?" Naruto asked, eyes searching. His voice was higher, more girlish. Even his facial features changed. His eyes were slightly bigger, his face less chubby, and all in all, he could out match Haku in looking like a girl.
"Yes," Haku answered. He looked Naruto over carefully and asked tentatively, "Are you a girl?"
"What? No, I'm-!" Then he looked over himself and let out a shriek, "How am I a girl? What the hell!" Her clothes were a lot baggier than they were a few minutes before the spike in chakra. And there was a slight, but definite, curve to her chest.
"Kakashi-sensei!" cried Sakura who was hugging Sasuke, "Sasuke, he-!" Kakashi was at her side, while Zabuza appeared at Haku's. He checked Sasuke's vitals and said,
"What?" Naruto gasped and turned to look at Haku, "You didn't kill him? Why?"
"Haku never liked killing. He never enjoyed it," replied Zabuza as he walked forward. Naruto immediately dropped to a ready crouch, but all Zabuza did was raise an eyebrow and said, "Relax, kid. I won't hurt you. Not now, anyways." Naruto kept her eyes on Zabuza, but stood up straight and went to Sasuke. She tried kneeling by him but ended up falling side ways.
"What the f-" she nearly cursed, but hit the ground before she could complete it, "Ow..." She attempted to get back up, but her muscles turned to jelly and she collapsed, the strain of the seal cracking taking its toll.
Her eyes opened and all she saw was darkness, and she heard dripping water. Her eyes slowly adjusted, and she found herself in front of a cage in a dimly lit cavern.
'Where am I?' She slowly got up and realized she was leaning against a wall, and was ankle-deep in sludgy water. 'Oh this is completely disgusting!' she thought, grimacing slightly at the sludge sticking to her pant legs.
She surveyed her surroundings, finding herself in a sewer-like room, in front of a cage door. Naru slowly approached it, trying to get a glimpse of what was inside. But then she stumbled back as claws passed through the bars of the cage, scraping at where she was a minute ago.
"Filthy pig! Release me this instant!" Before she could register the words, she retorted,
"Don't boss me around, you overgrown rabbit!"
I thought this was a good place to end off...the Kyuubi and Naru exchanging insults. The first step to bonding!