Tessaiga and Tenseiga have a serious, dignified origin story. This isn't it.
A/N: This was written before manga ch. 491 came out, so it doesn't fit with the latest canon knowledge we have of the two swords.
Fangs of our Father
The Inu no Taisho's stomach grumbled. Killing hordes of demon enemies sure worked up his appetite, especially since he'd had to do it all barehanded. He had yet to find a sword worthy of his stature, but right now his mind was on more important things-like food. He flew over the countryside in his giant canine form, scanning the fields for any livestock he could devour.
Finally, he came upon a land of noxious fumes, and spotted a bull demon grazing next to the entrance of a cave. How fortuitous, he thought, nose-diving toward the bull.
The bull's ears perked up, and it took off into the air at the last moment. The Inu no Taisho's claws bit the earth. He growled, zooming after it.
It swerved and ducked, but no one escaped the Inu no Taisho for long. With one swipe of his mighty paw, he had it in his clutches. He licked his chops, silently lamenting the lack of wasabi sauce at hand, but there was nothing for it. Eating the bull plain would have to do, but at least it wasn't a total loss. Cow was his favorite meat after all. Many a time had the shouts of humans rung in his ears as he raided their fields for cow. He was notorious for it.
The Inu no Taisho opened his jaws, ready to swallow the bull demon whole, when a hoarse voice cried out from the cave.
"Stop!" it said. "Don't eat my poor Momo!"
Momo? What kind of person would name their bull "Peaches"? The Inu no Taisho pondered this as a wiry, old demon ran out brandishing a long hammer in the air.
"That's my Momo! Put 'im down!" the demon said.
Never a moment to eat in peace. The Inu no Taisho sighed inwardly. He landed on the ground, and took his human form, which was no less impressive than his canine one, tall and imposing. He grasped one of the bull's horns in his hands, not relinquishing it. The old demon bowed at his feet. "Now see here," he told the demon. "I caught this bull of my own accord. By right he is mine, and I am very hungry."
"Please, noble lord. Momo has been my friend and companion for many centuries. Spare his life, and I shall grant you my eternal thanks and servitude." The old demon pressed his forehead to the ground.
For a moment the Inu no Taisho appeared moved. "I feel for your plight," he said, holding his stomach. "But I'd rather eat the bull." And he raised a claw to slice off its head.
"Wait!" the demon said, his hammer shaking in hand. "Is there nothing I can offer you in return for my bull?"
The Inu no Taisho scratched his chin. "Is that a blacksmith's hammer you wield?"
"Yes, my lord."
"Tell me, are you a good blacksmith?"
"The best, my lord, in all the land."
The Inu no Taisho's eyes gleamed. "You must make me a sword."
"A sword? Why, that's easy."
"It must be an extraordinary sword. One fit of my awesome power. A sword that could take down one hundred enemies with a single stroke."
The demon gulped. "With all due respect, my lord, such a sword would be nigh impossible to craft..."
"Do it," the Inu no Taisho said. "Or I shall eat your precious Momo on the spot."
The demon shook in fear, and prostrated himself once again. "Of course, my lord. Consider it done." He leapt up and surveyed the Inu no Taisho from top to bottom. "For a demon of your stature, I'll need a fang."
"In order to imbue the sword with your power."
"Oh, very well." The Inu no Taisho yanked one of his own teeth out, and sighed. He chucked the fang carelessly at the old demon, who cried out as it almost slipped through his fingers. He had never seen anyone withstand pain with such nonchalance.
The Inu no Taisho turned to go. "You have one week."
"A week? But my lord..."
"Silence. Now, tell me, blacksmith, what is your name?"
"My name? It's Totosai, but..."
"Well then, Totosai. I shall return in precisely one week. On that day you will fill my hand with a sword, or I shall fill my belly with your bull."
With that, the Inu no Taisho transformed into his giant canine form, growled once, and disappeared into the skies.
Totosai fell over onto the ground, eyes streaming. Momo's three-eyed face came into view as the bull prodded its master's head gently, and mooed.
"Oh shut up, you," Totosai said, pushing its face away. "This is all your fault." And he stalked off into his cave to get to work, fang and hammer in hand.
One week later, the Inu no Taisho returned, and he was in a foul mood. His stomach rumbled loud enough to startle frogs off their lily pads and send rabbits burrowing deeper into their holes. He still hadn't had a decent meal. He half considered calling the deal off and just eating the damn bull, but if Totosai was as good a blacksmith as he claimed to be, this might be his only chance to get a truly spectacular sword.
"Totosai," the Inu no Taisho called out once he had landed and taken human form outside the demon's cave. "I come to honor our agreement."
"Come in!" Totosai's voice filtered out from the back of the cave, where a screen of smoke blanketed him. He was hunched over something, spitting fire from his throat. "I'm just putting the finishing touches on your sword."
"Excellent," the Inu no Taisho said, making his way inside. Now that it came to it, he was quite glad of their arrangement. "You have no idea how many blacksmiths I've tried before. No one seems able to craft what I want. And so I remain unarmed until I find a sword worthy of bearing."
"An honorable choice. One should never settle."
The Inu no Taisho laughed. "I am unable of it. The last sword I commissioned, I crushed into a thousand splinters with my own fangs."
"Oh." Totosai gulped. "And what became of its blacksmith?"
"I killed him in the most horrible way possible."
Totosai gagged, choking on his own flames.
"But don't worry, friend. That was only because he failed to deliver what he promised. I'm sure you'll have better luck!" The Inu no Taisho laughed, and Totosai laughed along with him, only the laughter sounded strangely like sobs.
When Totosai finished, he sheathed the sword. "There," he said. "All done." He turned around and presented it to the lord.
"Ah," the Inu no Taisho said, balancing the sword. "This feels a fine blade. Truly you are a master craftsman."
"I'll simply remove the sheath and inspect the sword itself..." the Inu no Taisho said with a smile. The smile soon turned to a frown. The blade was a dingy piece of junk, not fit for a peasant let alone a lord. Totosai coughed. "What the hell is this?" the Inu no Taisho said, strangling him.
It was hard to talk with the Inu no Taisho's claws around his throat, but Totosai managed to say, "Ich a swor."
"It's a piece of rust! I cannot slay one hundred demons with this! It wouldn't cut a flea on my back!"
"Jus ty ich, pleas," Totosai said as his face turned purple.
"Oh, very well." The Inu no Taisho released his grasp on the older demon's neck. "I shall try it."
"Great," Totosai wheezed, clutching his throat.
"But if it doesn't work, I'm eating the bull. And perhaps I'll use your bones to pick my teeth."
Totosai gulped as he and the Inu no Taisho (followed by Momo the bull) walked out of the cave.
The Inu no Taisho produced a staff from his hip, and struck the ground once. It emitted a high-pitched shriek, and scores of low-grade demons came streaming out of the sky toward them.
"Your staff can summon demons?"
"Of course it can. Who says it can't?" The Inu no Taisho leveled the sword in front of him. "You better hope this works." The swarm of demons approached. Suddenly the sword pulsed in his hands. What the hell was going on? He could feel a surge of power through the blade, and it transformed into a giant fang. The demon hordes screeched and wailed, surging toward him. He gave a yell, swinging the sword. A wave of power erupted from the blade, disintegrating the demons on the spot.
When it was done, the Inu no Taisho stood breathless, staring at his own amazing fang.
Totosai watched him in awe. He hadn't expected the sword to work! It was dangerous to craft swords of power for those who might misuse them. He had forged the sword to obey only someone with a compassionate heart who would use it to protect the weak. He never imagined that the Inu no Taisho was such a person. He had planned to hightail it out of there on Momo as soon as the Inu no Taisho was distracted. "The sword, my lord," he said now hopefully. "How do you find it?"
The Inu no Taisho turned to him, and sheathed it as it transformed back into a thin, rusty blade. He snorted. "Acceptable."
Totosai fell over. Acceptable? Acceptable? That was all he had to say about the finest sword Totosai had ever made?
"I shall call it Tessaiga."
"Very good, my lord," Totosai managed weakly.
"Yes, it has a nice ring to it. Tessaiga. It should come very much in handy when protecting my beloved Izayoi."
"So, you're done here?"
"And you won't eat my beautiful Momo?"
The Inu no Taisho laughed. "Of course not."
"Oh, thank goodness." Totosai breathed a sigh of relief. He was glad that was over.
"As long as you forge me another sword of equal power. One that can summon the lives of one hundred people in a single swing! Yes, I should like that very much. One can never be too careful when protecting one's human." The Inu no Taisho pulled out another tooth, tossed it at Totosai, and turned to go. "I shall return in one week. See you then!"
The wind from the Inu no Taisho's takeoff knocked Totosai onto the ground, where he twitched uncontrollably. One week. He had one week to create another sword of nearly impossible power!
Momo came over and licked his face.
"I should have let him eat you."