With House of The Dragon currently airing, it's only fitting that another one of George R.R. Martin's adaptations have leading characters to hold unofficial titles that define them more than any official one could. Lord Corlys Velaryon, for example, is known better by his moniker the "Sea Snake" due to his extensive maritime adventures and his personal ship's namesake.
Game of Thrones introduced a lot of viewers to Martin's knack for nicknames to the point that some would rather refer them to these "nicknames" than their trueborn ones.
The Mountain
Not many characters in the series have a claim to a name that physically describes them. That honor or dishonor is laid out for Ser Gregor Clegane and Lord Tyrion Lannister, respectively, but the latter's is more derogatory compared to the absolute terror and fear the former inspires. Of all the Knights of Westeros, Ser Gregor is one who quite literally towers over them all.
Standing at over 8 feet tall (according to the novels at least), Ser Gregor is literally described as unmovable force by those who've clashed with the fearsome knight and lived to tell the tale. This is why it's very fitting to be bequeathed such a nickname as "The Mountain" as no one can dare stand against him in an open brawl.
The Red Viper
Prince Oberyn Martell was easily one of the biggest highlights of season 4. His boldness in both fields of politics and pleasure was a welcome one as it gave audiences a dashing rogue who seeks to stir trouble in the capital and enact his long-awaited revenge. His main motive was to see those responsible for the brutal murder of his sister and her children be brought to justice.
His moniker of the "Red Viper" is a fitting one as it manages to perfectly combine countless traits of Oberyn's character, both figuratively and literally. Though his time at the Citadel did not end with him becoming a Maester, it would bear fruit as he was able to study poisons extensively and how to best equip them as weapons in his quest for revenge.
Littlefinger
Petyr Baelish is a man known for many things that would be deserving of a nickname. From his Machiavellian scheming to his numerous betrayals, these would not play a hand in what gave him the infamous title of "Littlefinger." This name would be derived not from his personality but of his "lowly" status.
Born to an insignificant Lord of the smallest peninsulas known as the "Fingers" in the Vale, Petyr Baelish was considered low nobility compared to those he affiliated with in his childhood, mainly the Tully's. Edmure Tully was the one to bestow the nickname "Littlefinger" upon him as an insult. Yet, Petyr mischievously accepted the nickname as a way to motivate his desire to climb the ladder to power and leave chaos in his wake.
The Blackfish
Brynden Tully may be the only fictional character in history to have granted himself a badass nickname and still walk away with his integrity intact. After numerous attempts by his brother and Lord to arrange Brynden into marriage alliances, Brynden refused each one. Angered, his brother labeled him the blackgoat of the Tully family, to which Brynden responded that he was more the family "Blackfish," given the Tully sigil.
What was originally meant to be a joke, the name Blackfish became so synonymous with the knight that he himself would claim that it was the only name people remembered him by. Because of this, he even marketed himself off as the "Blackfish" and made it his personal heraldry.
The Usurper
The Targaryen family had ruled over Westeros for countless generations, so when King Aerys II's cruelty had become too much, Robert Baratheon was the one to lead the charge in open rebellion. No matter how the history was written, Robert's Rebellion ended in victory by defeating the Targaryen forces at the Battle of the Trident.
With the Mad King dead and his children fleeing for their lives, the throne was left empty. So, it was agreed upon that it would only be natural for Robert to take the Iron Throne. Thus, he would forever be known as the "Usurper" after sitting upon the seat that had once been claimed by the mighty Targaryens for a thousand years prior.
Queen Of Thorns
Not many in the Seven Kingdoms could hope to match the silver barbed tongue of Olenna Tyrell, as her burns were legendary for their sharp and surgical accuracy upon whoever was on the receiving end.
It's only appropriate that the "real" power behind Highgarden would be given the title of "Queen." But it's the double entendre of "Thorns" that truly makes it an iconic name as it's meant to both represent her family's sigil of a rose and her knack for hurling insults.
The Hound
Sandor Clegane may quite possibly be the most cynical man in all of Westeros. His disdain for the hypocrisy he sees among the knights and lords is worn on his sleeve, especially given that even his monster of a brother earned knighthood. This in turn would cause him to refuse any offers of becoming a Ser.
His nickname of the "Hound" was mostly derived from both his house sigil and his occupation as Prince Joffrey's bodyguard. Yet, his infamy and the name associated with it would stick with him even after furiously walking off the job site. Though Sandor's efficiency in enacting violence against enemies gives credence to his animal-based nickname, it's more befitting of a "Hound" to protect rather than to attack out of malice.
The Red Woman
The Red Priestess of R'hllor, Melisandre is from head to toe a true representative of the Lord of Light and the primary color red. Her zealous attitude burns brightly as she convinced herself and Iron Throne claimant Stannis Baratheon that he is the true "Prince that was promised."
Though the religion of R'hllor isn't as prominent in Westeros as it is in Essos, this makes Melisandre stand out more in her red garb than blend in. The Faith of the Seven is still the dominant faith of the Seven Kingdoms, but that won't stop this deadly magic missionary as she traverses the continent with prophecies to relay and kings to sway.
Kingslayer
The Kingslayer may be the most badass nickname to grace any character in the series, only to be detested by the one who owns it. Jaime Lannister's claim to fame was forsaking the vows he took when becoming a Kingsguard in Game of Thrones, only to eventually murder the very King he was sworn to protect.
Ever since then, Jaime has had to live with the judgmental eyes shared by his peers. They looked upon him with disgust for killing the Mad King Aerys Targaryen II, despite said King's true nature. While imprisoned under Roose Bolton, the Lion revealed to Brienne his side of that infamous day when the King intended to burn the innocent population. His actions saved countless lives, but his oath breaking was deemed more criminal, which forced him to bear the name "Kingslayer" wherever he went.
The Mother Of Dragons
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Queen Daenerys Targaryen hosts a multitude of titles, ranging from Khaleesi to The Breaker of Chains, and she had no qualms about accepting all of these namesakes in her pursuit of reclaiming her birthright. But there was one that stood out among the others. "Mother of Dragons" is easily the most distinguishable because she was the first person to bring back not one but three Dragons back from the brink of extinction a century and a half prior.
Ever since Aegon's conquest, the Targaryen name has been synonymous with the winged beast. So, when dragons made their way back into the world, it caused a widespread mixture of both fear and awe for the one who "birthed" them. After losing her own child, Daenerys took on the three dragons as her very own children, making the title "Mother of Dragons" more literal than one would perceive.