Uther Pendragon
The etymology of "Uther Pendragon" has led to much debate. The name Uther first arrives in literature in Nennius' Historia Brittonum, when he says Arthur was called "in British mab Uter, that is in Latin terrible son, because from his youth he was cruel." As such, when Geoffrey of Monmouth was writing his History of the Kings of Britain, he may have simply misunderstood and wrote that Arthur was the son of someone named Uther. If this is correct, then "Uther Pendragon" is the Welsh Ythr Pendragwn--"The Terrible {or Fearsome} Head-Dragon {Cheiftain}." On the other hand, the name appears independent of Geoffrey, and so the mistake may go back to Nennius, who is in the wrong. Either way, someone is wrong, and the name Uther is either an appelation of the father or of the son. There was a historical Arthur mac Aiden, of the Dal nRiada, but this was in the sixth century, and thus too late.
From Geoffrey onward, Uther (whatever his real name is) is the son of Constantus and brother of Ambrosius Aurelianus, who is a historical warlord that fough in the Battle of Badon Hill. Welsh geneaolgies have him decended from Maxen and Conan Meriadoc, leading ultimately up to old Llyr.
Uther is said to have become king after his father and brother are treacherously killed by Vortigern and the Saxons. In response, Uther has Merlin bury them at Stonehenge, which has been brought over from Ireland. Uther is able to defeat the Saxons, bringing some measure of peace to Britain. He falls in love with Igrain, the wife of the Duke of Cornwall (later called Gorlois), and has Merlin transform Uther's appearance into that of Gorlois, so that the king could sleep with her, thus begetting King Arthur in a move reminicent of Zeus' begetting of Hercules. Gorlois is killed in battle (not unlike the biblical husband of Bathsheba), and so Uther is free to marry her. After Arthur's birth, Merlin takes the boy to Sir Ector, and Uther reigns for many years. Various accounts are given of his death, but the typical one is of a murder, usually a poisoning of the water supply by the Saxons, who had recently lost a battle to him. He is then said to have been buried at Stonehenge with his brother.
Uther's name occurs in early Welsh poetry, in the Black Book of Carmarthen's poem "Pa Gwr," when Arthur lists as his servant "Mabon ap Modron, servant of Uthir Pen Dragon." This Mabon may be the Welsh god of youth, or it may be an appelation for Merlin, who is the Son of the Mother, having no father. The Book of Taliesin has a poem "The Elegy of Uther Pendragon," and another poem "The Elegy of Madoc" refers to "Uther's Son"--which may be Arthur, but which may be Madoc, who is sometimes named as a son of Uther.
An interesting idea, that Uther may have had another son who is rarely mentioned in Welsh literature--it leads to the question of why? Just as the question as to why Arthur's sons are written out of the story. Why is that? Because of some sort of embarressing failure? Some sort of traitorship? Or were they simply someone's invention?