A very very long time ago, before the princesses were in power and even before Equestria was founded, there was a pony. His name is irrelevant, and it shall not be mentioned. This was so long ago that he was there when first contact with the dragons were made, when he was only a small colt. He was fascinated by them and began to devote his life to their study.
He never talked to anypony, constantly keeping to himself and researching the dragons any way he possibly could, memorizing their general anatomy, which differed greatly from dragon to dragon, along with all the works by the prominent scientists studying dragonkind. Due to his fanatical obsession, he was always alone and didn’t know anypony else, not even the other ponies he had classes with.
Eventually, this young unicorn realized he would need help with his ultimate goal, which shall for now remain unknown. He collaborated with a very small group of others, who helped him acquire certain materials and information that he needed. Finally, after years of practice, experimentation, and research, he finally perfected the spell he had spent most of his young life on.
He packed up everything he needed and left his home, knowing that he will not return unchanged, and possibly not even accepted into society. He headed out towards the homeland of the dragons, where the largest concentration of them lived. He was called a fool and warned to turn back, but he ignored such people and pressed on until he finally found the dragons.
He searched throughout all of the dragon populace, looking for the exact right dragon, one who had a suitable physical shape and mindset. After more years of searching, when he had almost given hope of finding the right one, he found him; the perfect dragon, essentially the dragon version of himself, with an equally strong fascination with ponies, although the also nameless dragon had not bothered trying to pursue the same goal as the pony since he had no magical abilities himself.
After discussing the pony’s plan and thinking about it for a few days, they reached an agreement. They both knew that there was no going back, and that this decision would permanently change their lives. After reaching their conclusion, the pony began to prepare to cast the spell that was his life’s work, one that would be banned from any futher research or use, and all the notes and research pertaining to this spell would be confiscated and locked away in the most secure place possible.
The spell that would force the bodies and minds of both beings together, destroying them both while creating a new one.
The pony prepared the spell; everything had to be exactly right, for even the slightest mistake could have horrible consequences. Once it was finally ready, he cast the spell. The pony and the dragon were forced together, altering the bodies of both and putting both minds together. The pony grew unnatural wings, the pupils of his eyes became thin slivers, like those of all other dragons, his mane settled into the style it is now, with one eye always covered, and he finally gained his cutie mark, 20 years after all others. The dragon grew horns out of his head, which gave him the ability to use magic, his wings became more feathery, and he grew fur along his body.
However, the combination had some devastating effects as well. The being lost all magical abilities, completely drained of the substantial power he once had. It took this new creature weeks to recover physically, getting used to be able to transform between a pony and a dragon at will. It took even longer to get used to this new mind, with the minds of the pony and the dragon put together and communicating with each other through thoughts.
As he was getting used to this new body, he took some time to examine a couple of the changes as well. His new mane, which was once wild and disorderly from lack of care, now covered half of his face, and left the other half bare. His new cutie mark, a yin-yang sign with a pony on one side and a dragon on the other, combined with his mane, let him realize his talent and new duty. He was now fully two beings in one, and he had to maintain the balance between the two sides at all time in order to keep harmony within himself. Later on, however, he will discover that it is more than just that.
Once he had finally regained full control of his new existence, he emerged from the cave he had been hiding in. He attempted to talk with the dragons he used to know, but the shunned him, calling him an abomination and a freak of nature for unnaturally fusing with an inferior and weak pony. Failing this, he traveled back to pony lands, hoping to be better accepted there.
When he found himself back in pony-inhabited areas, he attempted to talk to ponies there, starting with his associates back in his pony hometown. Even though he had never talked to most of the people there, they knew who he was and he was rather infamous. After he had left, one of the ponies he had worked with, none of which had known his true intent, discovered the writings he had left behind. Realizing what he had intended to do, that pony spread news of it all throughout town, causing the ponies there to hate him for the same reasons the dragons did.
With some difficulties, he was able to obtain a pair of glasses that hid his eyes so that ponies that did not know him wouldn’t know what he had become. After receiving this final gift, he left to live in isolation for a long time, his lifespan extended beyond pony years due to the longer life of the dragon.
He watched from afar as Equestria was made, uniting the normal ponies into one country. He saw when Discord invaded, wreaking chaos throughout the land. Having a bit of a trickster side to him, along with a slight need for revenge for being essentially banished from all societies, he joined in with Discord, enjoying all of the madness the ponies suffered through. Eventually, the Princesses of the ponies, Celestia and Luna, found out about Discord, who turned him to stone. The unfortunate creature was discovered by Celestia and Luna, who brought him in to be punished for his crimes against nature.
He was captured by the new royal guards of the princesses, and instead of fighting, he went willingly, feeling he deserved whatever punishment he saw fit. Celestia, the main princess in charge, originally planned to have him executed in order to wipe the abomination that he is from the face of the world. However, Luna managed to convince Celestia to change the punishment, proposing that a different punishment might be better. Celestia then chose to banish him from the land instead, but he just smirked, telling her that it was pointless since he essentially already was.
Luna, knowing what it was like to be excluded and unappreciated in a similar way due to nopony paying attention to the night skies she tried so hard to make, suggested a deal be worked out with the dragons instead. The princesses decided to contact the Elder Dragon, the most powerful dragon and spokesperson for all dragonkind. After several days of negotiations, they decided on a new punishment for him, one that would determine the rest of his life.
Their new punishment was that he shall now serve as the mediator between all ponies and dragons, for as long as he lived. He saw this as a good deal at first, knowing that his life was nearly over anyway, having already lived for a couple hundred years, nearing the end of even his dragon-enhanced lifespan. However, he was then informed of a new condition. He shall be granted eternal life, restored to a youthful body and not suffer the effects of aging. He could not die from natural causes, the only way being killed by outside sources, and he could not even kill himself, for that would allow him to escape punishment too easily. He was granted the new name of Mar, to symbolize the abomination he is, and his pony and dragon forms were branded with red stripes to tell him apart from other ponies and dragons.
He was released from custody and sent out of Canterlot, with nothing but the glasses he always wore. As he left, Luna chased after him. Knowing that he had no magical abilities anymore, she offered to train him in the ways of darkness magic. Mar hesitantly accepted her offer, and studied with her for a while whenever he had time. Eventually, he was able to regain some of his magical abilities, knowing a simple healing spell, able to conjure up shadows, summon tentacles made of shadows, and manipulate his dragon breath to be made of shadows instead of normal flames. During his training, Luna also helped him realize the full extent of his cutie mark; not only did he have to keep the balance between his pony and dragon sides, but he also had to keep the balance between all ponies and dragons, and without this crucial balance, war could break out and only one side would remain.
After several years of training, Luna finally saw fit to give him one last spell; the ability to change his form into pure shadows. Feeling she did not have the many more years it would take to train him how to do so, she directly infused the knowledge to him by magically linking their minds. Not even a month later, Luna transformed into the terrible Nightmare Moon, and was banished to the moon for 1,000 years. With his studies at an end, Mar went back to living a solitary life, not making contact with anyone else, pony, dragon, or any other species.
After guarding the ponies and the dragons from each other for nearly a thousand years more, he was contacted by Celestia to fulfill a very unique request. She required him to go back to the Dragon Lands and try to blend in long enough to acquire several dragon eggs for her new Magic School. Doing as she asked, he brought her the needed dragon eggs, then left to live alone again.
After a few more years, he watched as Nightmare Moon returned after her long banishment, and kept watch over the small group of ponies that went to stop her, preparing to intervene if his former master could not be defeated by them. However, they were successful, and he went back into hiding.
Shortly afterwards, a dragon moved into the caves near Ponyville. He watched again as the same group went to get the dragon to leave, again ready to intervene. However, he did not need to, and as the dragon was preparing to leave, he seized the opportunity he noticed. Mar stopped the dragon before he left, and demanded the dragon’s entire hoard as the usual punishment for attacking and nearly killing the ponies. The dragon reluctantly obeyed, and flew away from the cave. Mar kept this cave for his own home, transfering the rest of all the hoards he had gotten from his job into that cave, and started to excavate and expand the caves to hold all the varying treasure he had collected over time. From this new home, he continued his watch, now closer to ponies than he had been in a long time. He witnessed the return of Discord, but was preoccupied with a small dragon uprising and was unable to visit his old friend. Mar also paid close attention to the young dragon living in Ponyville, preparing to stop him as a last resort during his short rampage, and he was at the ready to protect the young dragon and the ponies he was with when he ventured into the Dragon Lands.
Now, after thousands of years, he is starting to attempt to rejoin pony and dragon society, emerging from his cave to talk to them once again. While he is still shunned by most, the legends of him that had been passed down since his fusion scaring many ponies away, there are a few willing to talk to him, and he is starting to feel accepted for the first time in his life.