The legendary King Arthur was an Iranian

There are many evidences that King Arthur was not a mythological figure and existed in reality. In that case he was of Iranian (Sarmatian) Origin.
Below some of many sources

Sarmatia

The territory of Sarmatia was an expansive stretch of land reaching from the Caspian Sea in the East to the Vistula River in the West, and as far south as the Danube. Essentially, Sarmatia was a collection of independent tribes, much like ancient Germania, that encompassed parts of modern Russia, Ukraine, the Baltic States, Central Asian nations and into central European countries such as Romania and Poland. The Sarmatian people were a blend of Iranian nomadic horse tribes that were likely related to the Scythians. Herodotus suggested in the 5th century BC that the Sauromatae, perhaps the original Sarmatians, were descended from the Scythians and the Amazons.
The Amazon legend was widely accepted among Greeks and later Romans, thanks to Sarmatian women having a much higher social standing than their Mediterranean counterparts. Regardless, Sarmatians moved west from the Central Asian steppes and into Europe between the 5th and 3rd centuries BC. These migrations brought them into direct contact with the Greeks, who at time proved to be adversary and friend. Some Greek coastal towns paid tribute to the violent horsement, while others traded and held alliances of varying degrees. These alliances helped the Sarmatians completely overtake lands previously held by the Scythians and they disappear from history for the most part.
By the first century BC, Sarmatians came into direct contact with Rome through Mithridates VI of Pontus. In the employ of the Pontic King, the Sarmatians ran helped bring Asia Minor under his rule, and likely wreaking havoc in Greece and the Balkans, at the expense of Rome. These alliances would eventually be crushed by Pompey and by Caesar in the mid 1st century BC, but the Sarmatians would continue to be a threat to Rome for another several centuries. External pressures from marauding Huns and other eastern people pushed the Sarmatians farther west. The Iazyges, certainly the most commonly known tribe to the Romans, settled along the Danube, between Dacia and Pannonia, soon to be in direct conflict with Rome.
Initially, the Iazyges were cautiously welcomed by the Romans, as they caused problems for tribes in Dacia, but eventually they would ally against the common foe. The Roxolani, another Sarmatian tribe, had settled the region and joined with their cousins as well. By the early 2nd century AD, the Emperor Trajan led a massive campaign to conquer Dacia, and between 102 and 106 AD, he brought this region and these tribes under Roman rule. Just a generation later, under Hadrian, it was deemed more advantageous to allow the nomadic horsemen their freedom, though Dacia itself was kept under Roman dominion. Another generation later, the Sarmatians, now including the Alans who migrated all the way from the Caspian Sea, had joined with Germanic neighbors, mainly the Quadi and Marcomanni. Marcus Aurelius, in a series of bloody and protracted wars from the 160's until his death in 180 AD, eventually pacified the region, but this too would only be temporary. It's also during this point in history that the first 'Sarmatian Knights' or auxilia were moved to Britain to serve along Hadrian 's Wall.
By the 3rd Century AD, political upheaval in the Empire and continued unrest among northern tribes, brought the Sarmatians back into permanent contact with Rome. They occupied Dacia, which was largely abandoned by the Legions, and began to settle more permanent homes, acting as buffers with migrating Germanics for the rest of the Empire's existence. A century later, many Iazyges were brought south of the Danube, and into the Balkans, under Diocletion and Constantine as farmers and tradesmen. Those Sarmatians who remained in the greater expanse of land to the north and east were eventually overrun by Huns and Goths and were either destroyed or absorbed by the 6th century AD.
In regarding the Sarmatians, it's important to note their potential contributions to the lore and mythos of western civilization. Their foundation and relationship to the Amazons has already been alluded to, but their transfer to Britain has helped feed speculation on the origin of King Arthur. Lucius Artorius Castus, a Roman cavalry officer serving in the 2nd century AD has often been associated with one possible source of the true historical Arthur. Whether true or not, the Sarmatian contribution to the story is certainly one major piece of the huge Arthurian puzzle. The service of Sarmatian cavalry, from the 2nd century until the 5th century and the Roman withdrawal from Britain, along with the deeds and exploits of Artorius, may have allowed his legend to grow and foster with each successive generation of Sarmatian 'colonists'. They also provided an invaluable contribution in post-Roman Britain, fending off Saxon invasions, which certainly helped foster the growing Arthur mythology.
http://www.unrv.com/provinces/sarmatia.php

C. Scott Littleton and Linda A. Malcor's
From Scythia to Camelot
by
Victor H. Mair,
University of Pennsylvania

The story of King Arthur and his Knights of the Round Table is perhaps the best known legend in the world: witness the widespread infatuation with Camelot some thirty years ago. When I was a student at Dartmouth College about the same time, I was inducted into an old honor society called Casque and Gauntlet. In a most solemn ceremony, each member of the society was bestowed the name of one of the figures in the legend and thenceforth we steeped ourselves in the lore of that merry band of knights (we thought we were being very English). Casque and Gauntlet continues at Dartmouth and there are similar societies on other campuses. As another indication of how much alive the legend is in our own time, I recently saw reported in several major newspapers wire service accounts of the announcements by a lay religious order in Italy and an amateur historian in England that they each possessed the Holy Grail!
.....
Yet the authors of this magnificent book prove irrefutably that the very heart of this beloved cycle of legends, not to mention many of its telling details, derives from ancient Iranian peoples whose original home was the Eurasian steppes.
"Scythian" is a vague, catchall term for the Iranian-speaking peoples who ranged across the Eurasian steppes during the first millennium B.C.E. We do not know the specific names of all the tribes involved, but it is a scholarly convention to refer to them collectively and loosely as "Scythian." The best known among them were those who lived around the Pontic Steppes in Classical Antiquity. They were celebrated for their hoards of exquisitely carved golden art in the animal style, their superb horsemanship, their elaborate armor and excellent metal weaponry, their skill as archers, their bravery and strength, and their nomadism. The brethren of the Scyths of Classical Antiquity (who roamed over the steppes northeast of the Black Sea and southwest of the Urals) were stretched out all across Central Asia to the borders of China. The Pontic-Uralian Scyths were succeeded in late classical times by similar Iranian groups such as the Sarmatians and, somewhat later still, by the Alans.
In striving to locate the precise sources of the obviously Iranian components of the Arthurian legends, Littleton and Malcor rightly focus their attention on the lore of the Sarmatians, in particular the Iazyges tribe, and their kinsmen, the Alans. From archeological, artistic, historical, and linguistic evidence, we know that these northern Iranian peoples had a European appearance and that they were often blond-haired and blue-eyed. Temperamentally and culturally as well, they seem to have resembled Europeans in many respects. This is not entirely unexpected for Indo-European peoples who hailed from the western reaches of the Eurasian steppes. Certainly, they were very different from the Iranian peoples who moved south (the Persians and the Medes) and mixed with the indigenous peoples there. They had even fewer affinities with the later steppe nomads who came from the east and who spoke entirely different languages (the Huns, the Turks, and the Mongols).
The last surviving remnants of these ancient northern Iranian peoples are the Ossetians. The Ossetians are now living in the Caucasus Mountains, whence they were pushed by Mongols and other nomads from the east. Though threatened politically, militarily, and culturally from many directions, they still maintain their surprisingly archaic Iranian language and with it a body of oral narrative referred to as the Nart sagas. The Nart sagas constitute the best repository of the ancient western Scythian narratives that were transported to Britain and Gaul from the Pontic-Uralian steppes by peoples such as the Sarmatians and the Alans during the declining days of the Roman Empire. The Nart sagas contain parallels with Arthurian legend so numerous and so uncannily close that it is impossible they are unrelated.
The authors begin their argument with the archeological underpinnings that reach back into the second millennium B.C.E. They then move on to records of classical writers such as Herodotus and Ammianus Marcellinus. Through such sources, they follow the movements of various groups of northern Iranians as they spread out from the North Caucasus and the Pontic-Uralian Steppes. Moving outward, these groups impinged on surrounding areas, including the Mediterranean littoral and Europe. In the course of their wanderings, they came into contact, and eventually conflict, with the Romans. A key battle is that in which the Sarmatians were defeated by the forces of Marcus Aurelius. This was the so-called Marcomannian War of 175 C.E., after which 5,500 Iazyges were forcibly sent to Britain as armored auxiliary cavalry, primarily to bolster the defenses along Hadrian's Wall. It was this large infusion of Sarmatians that brought the first layers of the Arthurian cycle to Britain. The overwhelming majority of these fine warriors never returned to their homeland, but remained in veterans' settlements such as the one at Bremetennacum Veteranorum, a major Roman cavalry post near the modern town of Ribchester in Lancashire.
The next major northern Iranian inputs to Arthurian tradition arrive with the Alans. There is no doubt that the Alans were Iranians. Indeed, their name is probably a phonological transformation of the name "Iran" itself, which is in turn a variation of "Aryan". (It is worth noting that the major dialect of the Ossetians, Iron, is yet another permutation of the same word which means essentially "noble".) The authors provide an extremely detailed study of the historical sources concerning the Alans in Europe. This diligent spadework should convince all but the most obdurate xenophobe that not all of European history and culture was locally self-generated. It should also serve as a desperately needed model for practitioners of cultural history in other parts of the world.
In the course of their various chapters the authors are able to provide, in whole or in part, a Scythian (i.e., northern Iranian) basis for most of the prominent elements of the cycle, including:
The figures of Arthur, Lancelot, Perceval, Galahad, Gawain, Kay, Tristan, Bedivere, Bors, Caradoc, King Ban de Benoich, the Maimed King, St. George, Elaine (Helaine), the Lady in the Lake, and others.
The centrality of swords.
The throwing of the hero's sword into a body of water which causes it to seethe.

The Sword in the Stone episode.
The magical cup/cauldron that never runs dry and appears at feasts before the bravest of the heroes. The authors' account of the history of the grail legend in east and west is a scholarly tour de force. In their minute investigations of this theme, they adroitly and felicitously examine a vast variety of source material.
The frequent dragon/serpent imagery and themes.
The connection between women and snakes (e.g., the Lady of the Lake who is euphemistically referred to as the White Serpent).
Women as warriors and their association with water. Greek and Chinese legends about Amazons were undoubtedly inspired by hearsay about female warriors among the Scyths and later nomads who fought alongside their men.
Littleton and Malcor have demonstrated beyond any reasonable doubt that the underlying core of the Arthurian cycle is northern Iranian. They do not, however, claim that the Arthurian cycle is solely and exclusively Scythian. Several vital figures are transparently Celtic, such as Arthur's father Uther Pendragon (in Welsh, the name means "Glorious Head of the Troops") and, as we shall see below, his queen Guinevere (Irish Finnabair ["Born on the White," cf. Aphrodite = "Born on the Foam"]; Welsh Gwenhwyfar ["White Phantom"]). Thus, the main overlay to the tradition is Celtic, and there are also Christian, Germanic, and other ingredients. Since this convincing conclusion is so at odds with common wisdom concerning the tradition, it will not be easy either for the academic community or for the populace at large to come to terms with it. Yet we must accommodate ourselves to this new understanding, for to ignore the evidence would be to distort history, and the distortion of history is always a dangerous proposition.
By no means was the influence of Iranian myth and legend limited to Britain. Far away in East Asia, I have discovered clear evidence that some of the same unmistakably Iranian motifs found in the legend of King Arthur also turn up in old Chinese stories. For example, in an eighth-century popular tale about a sixth-fifth-century B.C.E. hero named Wu Tzu-hs�, we find the following passage:
Then he threw the sword into the river. It shot forth a spirit-like glow, sparkling brightly as it thrice sank and thrice came to the surface with a great gush and then hovered above the water. The god of the river far off heard the sword's roar and tremulously, he roiled the waters in a great and frothing frenzy. The fish and turtles were thrown into a panic and burrowed into the mud. Dragons raced along the waves and leaped out of the water. The river god held up the sword in his hand and, frightened, told Wu Tzu-hs� to take it back (Mair 1983:141 and 286 n. 699).1
Readers of Malory's Morte d'Arthur will instantly recognize the similarity of this extraordinary description with the last mention of King Arthur's Excalibur in that memorable romance.
Once one is intrepid and industrious enough to follow up the threads that beckon, one begins to grasp the interrelatedness of all cultures. The logical conclusion to this approach is that we need to adopt a genuinely global view of history, otherwise our history will be false. If we continue to view the development of cultures as though they arose in utter independence and isolation, we not only falsify history, we deny the bonds that join all humanity.
When I first read through From Scythia to Camelot, I found it to be absolutely riveting. Seldom have I encountered a work that is so well balanced in its presentation of stimulating ideas and substantial data. Perhaps because they were fully aware of its controversial nature and the fact that it would be strongly resisted by those who take a more narrow approach to cultural history, they carefully weigh counterarguments to each step of their own argument. Furthermore, in another refreshing display of scholarly honesty, what is speculation they label as such. For example, even though they have excellent grounds for their assertion that the prototype of King Arthur may have been Lucius Artorius Castus, a Roman commander to whom the Iazyges were assigned in their task of guarding Hadrian's Wall in the late second century, they do not claim this as an established fact. (The paperback, however, puts forth Castus's case more forcefully than the hardcover version did.) They are more confident that the "real" Arthur was one Riothamus, a shadowy fifth-century "High King" of the Britons who engaged in military adventures in Gaul.
...
The authors rely on the best manuscripts and are thoroughly conversant with the relevant primary and secondary sources for the Arthurian cycle throughout the course of its development. The book is scrupulously documented without being pretentious and pedantic. Mercifully, Littleton and Malcor eschew all jargon from whatever discipline.
In a dazzling display of classical erudition and oriental learning, the authors patiently provide mountains of pertinent information. Above all, what makes this book convincing is the massive marshalling of data and the explicitness of the methodology adopted.
Among the hallmarks of their work is the keen attention Littleton and Malcor pay to the etymology of names. One of the most crucial and daring that they propose is *(A)lan(u)s--Lot for Lancelot. Although not entirely convincing at first glance, given the overall argument and the mass of data (geographical, historical, and literary) that supports it, this etymological explanation of the name actually turns out to be rather persuasive.
Littleton and Malcor provide three substantive appendices. The first is a note on sources. The second is a series of stunning genealogies, including The House of Charlemagne (a relative of Goeric [clearly an Alan name, Goar, with an Old Germanic ending for "king"] of Albi), The Historical and Legendary Houses of Cornwall and Brittany, The House of Constantine, The House of Orkney, and The House of Joseph of Arimathea, all of which show Alan blood. The third is a discussion of Lucius Artorius Castus's career in Britain compared to Nennius's list of Arthur's twelve battles.
The authors do not pretend that they have created this new interpretation entirely on their own. To be sure, they recognize that they are building on the earlier research of scholars such as Jo�l Grisward, Georges Dum�zil, Bernard S. Bachrach, Helmut Nickel, Jean Markale, Jehangir Cooverjee Coyajee, and J. P. Mallory. What Littleton and Malcor have uniquely done is bring together a huge amount of relevant information from a mind-boggling array of sources and forged it into an irrefutable argument concerning the fundamentally Iranian nature of the Arthurian cycle. This is an extremely impressive accomplishment, one that deserves our highest accolades.
.
http://www.mun.ca/mst/heroicage/issues/2/ha2tf.htm
see also

http://www.hungarianquarterly.com/no144/p113.html