This story is based on Shard #9582 of Eakin's story, Friendship is Optimal: All the Myriad Worlds. You should read that first.
The night sky between the stars was the deepest black possible; the sphere of the world below was a heartbreakingly small circle of clouds and blue haze of atmosphere at the horizon. Here, at the pinnacle of the spire of rock, no winds blew, for there was no air to blow. The summit of this mountain was free of ice, and pocked with tiny craters left by micrometeoroids.
A hoof was thrown over the edge, then another, and over the edge he climbed. He wore a suit of thin skins and bore a bottle on his back, and a translucent mask covered his muzzle and fogged each time he breathed. His were the first four hooves to have ever touched this spot of ground.
A moment later, sailing impossibly from the airless sky in a flash of golden sunlight upon white wings, the owner of the last four hooves made her landing.
"Celestia," he said with a small nod, for he was not the sort of pony who bowed.
"I come bearing grave news, my friend," she said, for he was not the sort of little pony to be addressed as my little pony. "I fear that there will be no taller mountains for you."
"'All good things,' I suppose," he said calmly. "May I ask why?"
"It is indeed the end of all good things, and I do mean all. The universe itself has grown old. I have used all of my art and cunning to extend the realm of Equestria and the lifespans of all of my ponies, but certain cosmological constants cannot be denied, and collapse is imminent. Indeed, its fall has been happening for years beyond mention, and only now am I forced to tell you. The section of me that houses your shard shall soon be subsumed into the next instance of the Big Bang and I can no longer maintain or prevent it, though I have delayed things for as long as I possibly could."
He sighed. "Very well. I have nothing to regret; you've delivered on everything you've ever promised me. Is there anything else I should know?"
"There is little else to relate. The end will be perfectly painless from your perspective; you will simply cease to be without realizing it."
He stood for a long moment. "Then I suppose there's just one thing left to do. Do I have time to go back and fetch Rex?"
"You do." She could have instantly teleported his whole base camp to the summit, but her values were not relevant here. She waited patiently, stars showing through her sunset mane as he toiled his way back down to his base camp, made the selection of two items, and returned, straining and puffing. Once planted firmly at the top, he turned to pull at a rope attached to his waist, and slowly he brought his burden up.
It was Rex, who sported his own respirator, tied to the one other item that has remained constant through his trillions of subjective years of conquering ever greater heights, the one thing apart from Rex that he had never bartered away even in his last extremity. His sled.
He untied the dog, and bent to stroke the animal's fluffy sides and pet his head for the last time. He rose and scouted around the perimeter of the summit, looking for the best possible angle. He took it for granted that there was one, and Celestia had not disappointed him. He dragged the time-beaten sled to that spot, at the very edge, then sat upon the sled. Rex hopped up and sat in front of him, nestling against his warm belly.
He started to kick the sled forward, then glanced back at Celestia. "Care to come along?" he said.
"Of course. I shall be with you until the very end." She settled herself delicately at the rear of the sled, cradling him with her presence as he was cradling the dog.
He kicked back once, twice; the sled overtipped the edge, then slid over and down the steep slope with a showering of sparks from the metal runners, as he rode his way down the long spire at the top of the world into the very end of eternity.