Image is by samanthadoodles on tumblr. She's graciously allowed me to use her image for my fic and you should go check out her blog cuz it's awesome.
In my headcanon, Baby!Toothless is a newborn who got separated from his mother, possibly by a snowstorm. Toothless can't fly yet, but he can still wander around on his feet which is probably how he got lost in the first place. Elsa takes him in, feeds him, and keeps him warm until he's about the size of a small pony, and then he's just too big to hide from Elsa's parents, and he can fly so... they say goodbye and Toothless flies north to try and find his family. Then he gets caught by the Red Death, and HTTYD happens. It's pretty adorable in my head, so I'm going to post intermittent one-shots, probably.
Elsa found the little thing curled up beneath a dead log, shivering and half-dead in the cold. It was a dark winter night—safe for her to get some fresh air without the possibility of running into Anna on the grounds of the palace or being seen from her bedroom window. It was positively freezing, but Elsa eschewed the heavy woolens, scarves, and gloves she usually had to wear to keep up appearances. They made her hot and itchy and she didn't really need them but—she crouched down to look at the black animal that was freezing to death in the snow and she wishes she'd at least brought her coat to wrap the poor thing in.
She reached out, gently brushing the bumps of it's head dislodging the snow there. The rough texture of scales scratched beneath her fingernails, like the hide of the lizards that crawled all over the castle gardens the summer. The thing shifted, twitching slightly at the unexpected touch. Slowly, sluggishly, the animal opened its eyes.
"Oooh." Elsa breathed, getting on her hands and knees to get a better look at the thing's beautiful green eyes. Never in her life had she ever seen an animal with eyes so vibrant and… human looking. It was bizarre. She reached forward again, and this time the animal hissed and snapped at her fingers.
"Ow!" Elsa yelped, and cringed at how loud her voice was in the snowy emptiness. Her voice would carry out here, and she didn't want her parents getting angry with her for going out at night again-
But she couldn't just let some baby animal die out in the cold.
"Listen, you." Elsa said, firmly. "You'll die out here if I don't bring you inside. I don't care if you don't like me—I don't want my sister to find you out here tomorrow, stiff and cold. It'd make her cry, and I'm-" Elsa coughed. I'm the only one who can make her cry. "You come inside, I'll feed you, you get warm, and then we go our separate ways. Do we have a deal?"
She felt a little foolish, waiting for an animal to indicate understanding, but she thought maybe she saw something in those eyes that indicated intelligence. The black scaly thing huffed, unfurling itself and taking a few hesitant, slow steps toward Elsa. Now that it wasn't half obfuscated by snow, the princess could easily see what had been hidden before.
Dragon. It was a dragon. It was only about the size of a kitten, but a dragon nonetheless. She looked down in panic, eyeing the scrawny little wings and slender tail but mostly focusing on the set of teeth she could see poking out of it's mouth; could she get away fast enough before the thing attacked her, or called it's mother down to raze the castle to the ground. At the thought of a bigger, angrier mama dragon she glanced at the night sky nervously. She didn't think any of the Arendelle knights were qualified to fight dragons and she really didn't want to have to explain to Father how she'd accidentally caused a dragon attack—
Something warm, slimy, and wet clamped around her ring finger. Elsa jumped, and looked down. "What the-" the little black dragon was gumming her finger with a mouth without teeth. T"I swear you bit me earlier. And I know I saw teeth." She muttered, her heart rapidly melting under the crooning sounds the little reptile was making. It was… adorable. It almost looked like it was smiling at her, with its head tilted just so and its lip curled upwards slightly.
She reached out with a tentative hand, and the dragon nuzzled her hand, leaning into the warmth of her palm and purring. It'd been two years since she'd started avoiding Anna, and she was a very lonely ten year old; Mama and Papa wouldn't let her get a pet, afraid she'd kill it accidentally or something. But this…
"At least you seem to think I'm safe to be around, even if you're just using me." Elsa murmured. She gestured to her lap, and the dragon hopped into the warm cradle formed by her knees, settling down like it belonged there. "For my body heat, of all things." The irony was not lost on her.
The baby dragon snorted, tucking its head beneath its wing and curling tightly. It was still shivering.
Should I take it inside? She wondered, scratching at one of the crests on the things head for a second. Mama and Papa will be angry. They'd never allow it. A dragon, in the castle? No way.
But… Elsa smiled a little, gathering up the dragon in her arms and ignoring the slightly irritated grumbling of the little beast. What her parents didn't know wouldn't hurt them.