A/N: This is my take on how Vikings in the HTTYD world might interpret the origin of dragons, considering their old worldview, loosely based on real life Norse mythology.
The Origin of Dragons
Stoick had been telling Hiccup stories of their people. He was working his way through the Voluspa, and he'd recently told the legend of how Odin, Vili and Ve created the first humans, Askr and Embla, from tree trunks on the shore. The following night, his son asked "dad, if Odin made us, who made the dragons?"
"Well, funnily enough, that's the next story" replied Stoick, hoisting the little boy onto his lap. "So, the other gods were pleased with humans, all except for Loki. He thought they were dull and boring. He moved amongst them in disguise, spreading mischief and mayhem for his own amusement. Then Odin found out" he explained.
Hiccup's eyes went wide. "What did Odin do to him?" he asked nervously.
Stoick answered "The All-Father was very displeased, but he couldn't make Loki stop. Well, he could, but he wasn't supposed to. Loki said he was only fulfilling his role as a god of mischief. He insulted Odin's creations, so Odin insulted him in turn, telling him he could never create, but only destroy. Loki thought, 'I'll show them'."
"Loki had a wicked plan. He'd make his own new creatures to stand against Odin's humans, and let them cause chaos wherever they went. He decided to create dragons, because they would fly whilst humans walked, they would be grotesque where humans were handsome - and since the first humans were made from wood, Loki would make his dragons out of anything except wood, so they'd be fireproof."
The little boy on his lap inquired eagerly, "how did he make the dragons, dad?"
"Hold on, I'm getting to that. First, Loki took scales from Jormungandr, the Midgard Serpent, and as many materials from our world as he could find. He had to use things of Midgard, you see, or the dragons couldn't exist in our realm. Anyway, Loki took all this down to the bowels of Muspelheim, realm of fire and chaos. There he began."
"Loki used different materials for different dragons. He took the hide of Jormungandr and wrapped it around boulders to make Gronckles. He made Deadly Nadders out of arrowheads and spears. He filled two headed monstrosities with swamp gas to make Hideous Zipplebacks, and I bet you can guess how he made Monstrous Nightmares."
"Um, with fire?"
"Aye. Loki made all these deadly, hideous, monstrous dragons, and he breathed life into them, and he gave them fire, because Loki is a fire jotunn. Then he unleashed them on Midgard. They flew up from Muspelheim and over the waterfall at the edge of the world. They came from beyond the sunset, and they've been here ever since."
Ordinarily, Stoick would have ended the story then and there, but Hiccup quickly noticed "wait, dad, you missed one. What about the...the Night Furies?" he asked.
"Oh, now, it's getting late. How about I tell you that part tomorrow?" Stoick offered. Hiccup pouted until he relented. "Oh alright. So, Loki went back to his usual mischief, this time in Asgard. He pretended to have nothing to do with these new dragons that had emerged to wreak havoc. Odin and Thor imbued their human warriors, Vikings, with the courage and strength and skill to fight these wicked creatures" he went on.
"Then one day, Loki did something unforgivable. I can't tell you what it was, that's a story for another time. Before he was punished, he asked his daughter Hel to do him a favour, to create one more kind of dragon, one so powerful and cunning that no warrior could stand against them. Hel agreed, and he told her how to make them."
"So she took more scales from Jormungandr, but of course, everything Hell touches withers, and the scales did too. They turned as black as a shadow" Stoick revealed.
Hiccup's eyes were wide and round. "Then what did she do?"
"Hel made new dragons from these scales, but as Goddess of Death she couldn't bring them to life. Instead, she placed the black husks on top of the highest mountain in her realm, and sent one of her messengers to goad Thor in Asgard. Her message to him was, 'I bet you cannot strike this mountain here in my realm, God of Thunder.'"
"Of course, the mighty god Thor had to meet this challenge, so he hurled lightning down from Asgard. It couldn't hit the mountain in Hel, because Midgard was in the way. So he came down to Midgard and tried again, but his lightning still wouldn't strike the mountain. Finally, when he was frustrated and full of righteous anger, Thor came all the way down into Hel's realm, and struck the mountaintop with Mjolnir."
"The lightning struck them, and the new dragons burst into life, black as night and furious as a storm. Hel caught two of them to pull her chariot. She named them Leiptr and Dauthi - lightning and death, after what they were born from. She gave one of them to Thor as an apology, and now it flies with the Wild Hunt, chasing doomed souls. You can hear it's shrieks in the dead of night. The rest escaped into Midgard."
This was the most exciting part of the story. "Wow. What do Night Furies look like?"
Stoick admitted "none of us know. Nobody's ever seen one and lived to tell the tale. Some say that Hel made all of them her servants, so the only time you'd ever see one would be just before...well, you know." Hiccup shrank in on himself. "Ah, don't worry. No dragon's gonna get you, so long as you do as you're told and stay inside."
"Aww, but I wanna help fight the dragons!" Hiccup pretended to swing a sword.
His father chuckled. "You're too little, son. You can fight them when you're older."
"I heard that when you were a baby you pulled a Monstrous Nightmare's head off it's shoulders" declared Hiccup. "How come you weren't too little?" he asked impishly.
"I was a big baby," replied Stoick. His son giggled. "Tell you what, I'll ask Gobber to let you be his apprentice. Then you can help make the weapons that me and the other warriors use to kill dragons. Won't that be fun?" he asked cheerfully. Hiccup nodded hesitantly. "That's my boy. Anyway, it's high time we started getting some meat on your bones! You won't be tearing the heads off any Nightmares with arms like those."
Stoick laughed. Hiccup forced himself to snigger, but he didn't think it was very funny. It was bad enough when Snotlout made fun of him for having sticks for arms; he didn't want his dad to think he was a weakling too. "Alright, come on you, time for bed" declared Stoick, lifting his son off his lap. "Run along now, there's a good lad."
The years went by; for a long time Hiccup didn't give the legend of Loki and the Dragons a second thought. He forgot about it, in fact, and never stopped to question the wisdom of shooting down a supposed servant of Hel herself. Or of most things.
At last he achieved his goal, of capturing a Night Fury. When he first laid eyes on the dragon, Hiccup was terrified, but also intrigued. He'd always vaguely imagined a Night Fury as a vicious looking dragon, with long razor sharp claws and jutting fangs like all the other dragons had. Yet instead it was smooth, almost elegant, clearly built for speed rather than strength. "I did it" he murmured in wonder. "I actually did it!"
"Oh, this is amazing! I have brought down this mighty beast!" he crowed triumphantly, planting a boot on the dragons side. It twitched under him and he leapt back in shock.
It was still alive? He thought the fall and crash had killed it. Frightened once more, Hiccup gathered his courage and pulled out his knife. He stepped nearer to the Night Fury. His gaze travelled to its head, and he gasped. The dragon's eye was open, and staring at him. In its bright green eye, Hiccup could see his own reflection, but also fear. The offspring of lightning and death was scared of him, a fishbone with a knife.
Right. The knife. "I'm gonna kill you, dragon," Hiccup declared, trying to psych himself up. "I'm gonna...gonna cut out your heart and take it to my father. I'm a Viking. I am a Viking!" He raised the dagger high, preparing to stab it down into the dragons' chest.
The Night Fury moaned sadly (why did he think it was sad, dragons couldn't feel sadness) and looked away. It was accepting death. Hiccup was trembling all over. He had to do this. He had to...to kill this defenceless...frightened...oh, who was he kidding? He was a coward. Hiccup lowered his knife and stepped back. "I did this."
He had to set it right. Hiccup sawed through the ropes, praying that he wasn't making a terrible mistake. At last they fell away, and the Night Fury sprang to its feet. It pinned him against a nearby boulder, snarling. Hiccup's heart felt like it would burst out of his chest. Oh, now he remembered the legend. He was about to die, he just knew it, and Hel would probably feed him to Nidhogg for trying to harm her servant.
The Night Fury stared at him for what felt like eternity, and then it screamed right in his face and fled. Hiccup struggled to his feet, picked up his knife, and then fainted. It was not until Gobber said "dragons always, always, go for the kill", that he realised that maybe both his mentor and father were wrong, about Night Furies anyway. No one had seen a Fury and lived to tell the tale...until now, and he couldn't tell them.