Ever so slowly, Hiccup got up from his chair and made his way outside. Yesterday he had turned 60, which was extremely old for a man in those times. No one had expected the Chief to live this long, but he had, and he had had the best life any Viking could have.
Thirty-five years earlier on this day, his best friend had left with all the other dragons for fear of Drago coming back, and life had never been the same. For anyone. But Hiccup still had his beloved wife Astrid, his children and his grandchildren as a family.
Every year on this day, Hiccup walked down to the cove where he had first met Toothless, to remember him. He didn't know if he was even alive, but all he knew was that he was his best friend, his first friend, and he would always be a part of him.
But this time, as Hiccup made his way through the forest, he heard a strange but familiar noise. He couldn't make out what it was, because his hearing wasn't as sharp as it used to be.
As he got closer to the cove, he saw a black shape. It wasn't moving, but it wasn't dead, either. It was simply sitting there, peacefully watching over the cove.
The old man made his way towards it, and immediately thought that he was dreaming.
“Toothless?” he practically croaked in his worn voice.
The black shape turned around, to reveal two playful green eyes. It was him.
Even though he was still as black as night, some of his scales had turned grey from age. He had gained a few more nubs on his chin too. His saddle, still in place on his back, was worn from age and bleached light brown from the sun. His prosthetic tail was torn and dirty, and all the mechanisms Hiccup had made for it were old and rusty.
When Toothless saw his rider, the old puppy inside him came out and he bounded towards Hiccup and nudged him affectionately with his nose as if to say “It's not a dream. I'm here.”

(here comes the sad bit – if you haven't been listening to it already, listen to Stoick's Ship while reading it)

The next day, Hiccup Horrendous Haddock the Third, Chief of the Hooligan Tribe of Berk, passed away in his sleep. Everyone had known it was coming, for it was rare for someone to live to the age of 60 in those times.
He was sent out on a burning ship, just like his father had, with his human family and the rest of Berk watching from the beach. But Toothless wasn't there.
He wasn't there because, in that burning ship, there was another shape. Larger than a human, with big black wings and nubs along the sides of his chin. Toothless.
He had known Hiccup's end was coming. He had known it would be soon. So he had left the other dragons and had come to Berk, so that Hiccup could spend his last moments with him, his best friend. And seen as his skin was too old to be fireproof any more, the flames in the ship would affect him like it would affect his rider.
So in the end, no matter how long they had been apart, The Two Alphas passed away together.

end written by cometotheberkside:

Splinters of wood shot down as the ship burned, the blazing fire shimmering over the waves. The Night Fury onboard watched the ship break under the heat, wise eyes reflecting the orange light.

A large chunk of the mast broke off suddenly, falling towards him and his rider on the deck. As fast as he could, the dragon bounded over to his best friend, enveloping the late chief in his arms and wings protectively, as he once had done when Hiccup's fifteen year old self had fallen in the flames. As more flaming planks drove down hard onto the dragon and the boy, the embers bit into the hide of the dragon and made him writhe in agony.

It was all right, though, as long as his rider was safe.

He nuzzled the man's hair tenderly, remembering the memories they had shared as the white-hot fire crackled and spit. Already the world was fading out of his grasp.

He swore he felt the wind against his wings, ruffling them, and he felt calm all of a sudden.

I know I'm not so fireproof anymore, Toothless thought, as the world slowly collapsed into a dark mass, sinking into the murky depths of the sea.

But at least we're in this together.

a following end written by draco:

As the fire continued to eat away at the boat, slowly sinking into the ocean with Hiccup and Toothless on board, Astrid started thinking back to before they had first made peace with the dragons. She used to have to be the best at killing them, and nothing would have gotten in her way.
Then, Hiccup and Toothless had showed her a different side to them, a side that would change her life forever. Now, years later, as the chief's wife, Astrid, the villagers, her kids and grandchildren watched the boat sink into the ocean.
Suddenly, Astrid got the strangest feeling to turn around. When she did she saw, to her surprise, many dragons sitting along the hills of Berk, watching as the boat sank. Then, all the dragons bowed in honour and respect to the two that brought peace to Vikings and dragons after so many years of war. After a few minutes of silence, the dragons took flight and returned to the place where they lived for all those years, never to be seen again.
Astrid looked back towards the ocean, but the boat had fully sank and was no longer in sight. She smiled, and said to herself: “Until we meet again one day.”