- "The men of Ib are short, thick and hairy...But the Ibbenese know how to build ships: their great bellied whalers can weather any storm and withstand assaults of the largest beasts."
- ―Euron Greyjoy
The Ibbenese are a hardy folk from Ib, a large island nation located in the cold waters of the Shivering Sea, off the northeast coast of Essos (roughly north from Vaes Dothrak). Adapted to the cold climate, they are very short, thick and hairy. Due to their stocky builds they are very strong in battle.
Ironborn ships have been known to raid Ibbenese whaling vessels but they don't consider them very enticing targets: Since they don't carry expensive spices or gold like merchant ships from the Free Cities, the ironborn don't stand to profit much from attacking them, and since the decks and holds (and the crew themselves) have a powerful stench of whale blubber and oil, attempting to take the ships is an unpleasant task.
In the books
The Ibbenese have appeared in the background since the first novel, but were not described in great detail. They are most famous for their whaling ships that travel vast distances aross the world. The World of Ice and Fire sourcebook (2014) considerably expanded their description and backstory:
Based on their description, the Ibbenese seem to be a surviving pocket of Neanderthals, or some other closely related hominid. Human cultures across Westeros and Essos, however, do not think of them as a separate species, just a particularly odd-looking nation of humans. Apparently, due to contact in the distant past with explicitly non-human races such as the Children of the Forest and the giants, the human races of Westeros and Essos thought the Ibbenese were much more similar to them than to these races, and consider them "humans" under a broader sense of the term.
The Ibbenese also seem to match physical descriptions of Neanderthals - albeit keeping in mind that modern scientific reconstructions indicate that Neanderthals were not the dumb brutes they are often mythologized as: to survive the Ice Age, Neanderthals were adapted to be stocky and strong. Thus they were very powerful in hand to hand combat, but had less endurance for running or length of limb which aids throwing rocks or spears. As the Ice Age ended these advantages vanished, and they were generally outcompeted by biologically modern Homo sapiens migrating up from Africa (who were adapted to a post-Ice Age world in which smaller and faster animals needed to be chased down or killed with ranged weapons). In several respects this is comparable to the difference between regular wolves and direwolves, who were similarly much stronger than their baseline cousins because they were adapted to a colder climate with larger bodies to preserve body heat.
The Ibbenese in the novels are described sturdy and stocky but squat, very broad in the shoulders and rarely taller than five and a half feet. As a result, Ibbenese are among the best wrestlers in the world. Their heads have sloping brows with heavy ridges and small sunken eyes, and their massive jaws are filled with large square teeth. While the earlier novels consistently describe them as "hairy Ibbenese whalers"; the World book states that they are extremely hairy all over their bodies, the hairiest people in the known world. It is not outright fur, but covers their arms and backs; their men have large beards and even their women are quite hirsute. Underneath their dark, wiry hair, their skin is actually very pale white, from living in arctic climates.
A major hint at the nature of the Ibbenese is the statement in the World book that it is difficult to produce hybrid children between one Ibbenese parent and another parent from any other human race. It is said that the few who are produced are often sterile, in the manner of mules - hybrids between horses and donkeys. The ability to produce viable offspring with each other is usually the scientific basis for what constitutes distinct species. This is contradicted by a few points in the main novels, however, in which other maesters speculate that certain populations (such as on Skagos island) have a bit of Ibbenese ancestry in them - so it's possible that not every hybrid is universally sterile. This mirrors real-life debates about whether or not Neandernthals interbred with pre-modern Homo sapiens.
In their temperament and relations with other peoples across Westeros and Essos, the Ibbenese are actually not that different from other "human" races found throughout the world: on average no more or less greedy, war-like, or friendly. Some are cruel sellswords who go to war for money, others are friendly merchants who intermarry with local fishing communities from White Harbor and the Three Sisters to Braavos. By and large, most of them are infamously taciturn crews of whaling ships who stop in at major ports like King's Landing to resupply, and they largely keep to themselves - they don't like mingling with others but don't cause much trouble either. Most people in towns they visit don't want to spend more time with them anyway, given that it is repeatedly said that these crewmen have the foul stench of whale blubber about them. They are also described as somewhat miserly and stubborn: not in the sense that they are greedy merchants trying to acquire new gold (most are simple fishermen), but once they have earned money they don't like part from it. Infamously, their colonies on the northern coasts of Essos resisted the Dothraki for generations in the face of overwhelming odds, not so much due to honor but because they refused to pay tribute.
Not much is known about Ibbenese history, except when it crosses into the histories of other peoples - particularly their long wars against the Dothraki to the south. In ancient times the Ibbenese colonized a large swath of forest on the north shore of Essos, immediately between Ib and Vaes Dothrak. In the process, the Ibbenese seem to have driven the native "woods-walkers" to extinction (apparently an enclave of the Children of the Forest living in Essos, or perhaps their distant cousins). The Dothraki didn't dare attack the Ibbenese lumber settlements for centuries because they wouldn't enter the forest out of fear of the woods-walkers. During the great outward conquests of the Dothraki during the Century of Blood, however, they realized that the woods-walkers were by then extinct, and launched into long and devastating wars against the Ibbenese. Nonetheless, the Ibbenese put up sturbborn resistance, only yielding ground gradually. Their colony-cities on the mainland were burned, but the Dothraki never entirely drove them away. Fundamentally, the Dothraki would never cross the sea to attack the Ibbenese home island, so they could never defeat them completely. The smaller settlements which survive on the mainland today are much smaller and heavily fortified with wooden palisades. Chief among these is the settlement named New Ibbish.
Otherwise, the origin of the Ibbenese is shrouded in the mists of time. There are tales from oral history that during the Dawn Age there was a very similar race of men found in western Essos, known as the "Hairy Men" - though it is an open question whether they were the ancestors of the Ibbenese, or the Ibbenese were the ancestors of the Hairy Men. Whatever the case, the Hairy Men inhabited the northwestern regions around the Axe, Norvos, and Andalos, but were driven from each by the Andals during their migrations until they were wiped out.
Not much has been revealed about Ibbenese society and social customs. Their religion seems to have once included a line of God-Kings but they were later overthrown. The Ibbenese are quite crafty and intelligent engineers and shipwrights, capable of fitting together incredibly sturdy whaling ships without use of any nails, which can survive blows from the largest whales.
The Ibbenese have their own language, though they can learn other languages. No samples have been given beyond a few proper nouns, which tend to be short (i.e. an Ibbenese sellsword named Togg Joth). The Ibbenese language is described as guttural and grunting.
While few Ibbenese characters have ever been prominent in the novels, they are commonly encountered in most of the major port-cities of the world: from Braavos, Pentos, and Volantis to King's Landing and Oldtown, and even the Summer Islands. An Ibbenese whaler was even seen as a guest at Daenerys's wedding to Khal Drogo in Pentos, during the third chapter of the first novel. Some Ibbenese characters are also encountered as mercenaries, among foreign sellsword companies and the like. In the books, Tywin brings the infamous sellsword company known as the Brave Companions to Harrenhal, composed of the absolute worst dregs of humanity from across the world: among them are several Ibbenese axe-men. The TV series heavily condensed the Brave Companions into just a particularly vicious group of House Bolton soldiers led by Locke (their analogue of Vargo Hoat from the books).
It is unclear if the Ibbenese would ever actually appear in live-action TV episodes. To portray them accurately would at least require some mild prosthetics - though again, Neanderthals were not some kind of ape-men, they were just a little stockier than modern humans, with prominent brow ridges but weak chins. The most likely reason that they haven't appeared in TV locations that they did in the novels (such as Pentos or King's Landing) is probably just due to the same reason that entirely human races such as the Summer Islanders in King's Landing seem to have been removed: the writers either thought it was distracting or would take too long to explain due to limited screentime.