The Patriarch; the one who in ages past bred with the horsekind, fathering its own, new species, is now chained on the walls of a cave, never being able to escape; never able to die, the latter because it (he?) simply never -never- had the capacity for that. Even the very few -they could be counted in the fingers of one of their hands- humans who ever happened to find this animal and its prison could not fathom who might have chained the proud creature and how, though they did have their ideas on why; for the Patriarch was simply too powerful to be allowed to roam free, thought the human visitors - some of them, at least.
The young man approached the seemingly dying animal carefully, as to not attract its ire.
He knew that the creature was even more adept than phoenixes at regenerating itself (a bit slower, but with no period as a youngling like the one those marvellous birds went through), but he was still very sad at the atrocity his siblings committed simply to further their own ends, no matter how important they might consider them; too sad to ponder the fact that it would normally have fully regenerated, given as much time as this.
Of the brothers, he was the only one the mighty winged horse ever allowed near itself, and after it grew to trust him more, it even let him pet it during most of his visits. He sometimes thought he was able to understand the horse, grasp the general gist of what its cries wanted to convey, but it might have been his imagination.
"Did you know that the Ollivanders can use Thestral hairs as the core of wands?"
That was how the whole thing started, with this seemingly innocuous comment by the ambitious eldest. Soon enough, his other brother was persuaded to help, when the eldest hinted that the possible connection the creature had with death might be able to help with his own project.
The poor fool was obsessed -and pained and broken, the young man mused sadly- enough to go along with the plan. And no matter how much he tried to convince them it was a foolish one, the other two had never listened to the "seamstress" as they mockingly called him because of his hobby; so it was a given they wouldn't this time, either. The argument ended with him being placed in a body-bind and stunned.
The two of them attacked and injured the horse enough for it not to be able to strike back (body-binds, impediments and stunners were never enough to even wind the creature); so the one managed to pluck at least one hair and the other... possibly collected the blood spilled.
The young man rushed to the cave as soon as he was able to move. Obviously, he was far too late to do anything, so he could only speculate as to how exactly the event played out. And that was how he ended up in the position he was now.
Funny; he thought it would snap at him, that it would blame him for what happened. But it simply let him approach and pet it, as usual. Somehow, that made him feel even more guilty.
However, something far more extraordinary happened after he pet it. Its pelt and part of its wings were shed, much like a snake sheds its skin, and new ones appeared in their stead. That had never happened before, even after the one time when his brothers, in a drunken state, tortured the horse and inflicted injuries far more extensive than they did this time. Ignotus could only stare between the being and the discarded body parts in wonder.
His friend seemed to be gesturing towards the parts with its snout.
"You... you want me to take them?"
The horse made a sound which was likely affirmation. "But why?"
The next cry lasted for several seconds, and one could say it was the creature's version of a sigh.
"They are my brothers, even if they are major arses; they wouldn't really harm me! And I'm not worthy of these, either." Ignotus was certain that in any other circumstance, he would have questioned his own sanity for holding a conversation with his four-legged, winged friend. But he quickly pushed such thoughts out of his mind.
The horse-like creature clearly snorted.
-Hallow-Hallow-Hallow-
He ran away from the place he and his brothers had called home for years, so that when (if?) they returned, they wouldn't find him there.
For the last few months, he had slowly been working on fashioning a cloak out of the Thestral Patriarch skin. If he understood the creature's intent correctly, the cloak would help him hide from his brothers and survive.
He hoped that he understood it wrong, or that the creature's predictions when it came to his brothers were wrong, but Antioch's aggressive and ambitious nature in conjuction with his recently wavering sanity as well as the even worse condition, by far, of Cadmus's mind were points to the contrary. Not to mention the rumoured death of old Ollivander...
At this point, he had almost finished the cloak, but had yet to find a use for the wing part.
After a sudden inspiration, Ignotus decided to sew the wing bones into the cloak and make some sort of shape. He detached three of the four bones from the wing membrane and made a triangle using them; though it wasn't exactly even in shape, especially considering the fact that the bones weren't the same length... and that the cloak was barely big enough to contain them.
Finally, he put what remained of the parts inside the triangle. He didn't even bother detaching the last (and smallest) bone from the membrane. So, he ended up with a line inside a sort-of ellipse inside a triangle.
Ignotus was examining his handiwork proudly, when another miracle happened: the bones and membrane were absorbed into the cloak, the shapes became an even triangle, a clearly-defined circle which fit exactly right in said triangle and a straight line which fit exactly right in the circle and triangle. Then, the entire structure shrunk by a lot and became somewhat transparent.
When this occurred, Ignotus immediately knew that his brothers had succeeded as well, what the shape symbolised, and that even as he saw the strange shape forming on his cloak, the exact same shape was appearing on two other items, somewhere in the world.
"...I think it more likely that the Peverell brothers were simply gifted, dangerous wizards who succeeded in creating those powerful objects"
Albus Dumbledore, during one of the last few chapters of Harry Potter and the Deathly Hallows