Pendragon or Pen Ddraig (pronounced Thraig), meaning in Welsh "head dragon" or "chief dragon" (a figurative title referring to status as a leader), is the name of several traditional Kings of the Britons:
Ambrosius Aurelianus, son of Constantine II of Britain, called "Pendragon" in the Vulgate Cycle
Uther, brother of Aurelius and father of King Arthur, is called Uther Pendragon because he was inspired by a dragon-shaped comet (In the Vulgate, he took the name from his brother)
King Arthur, son of Uther
Maelgwn of Gwynedd, described by Gildas as the "dragon of the island"
In the Historia Regum Britanniae, one of the earliest texts of the Arthurian legend, only Uther is given the surname "Pendragon", which is explained as meaning "dragon's head". In the prose version of Robert de Boron's Merlin, the name of Uther's elder brother Ambrosius is given as "Pendragon", while Uter (Uther) changes his name after his brother's death to "Uterpendragon".
The use of "Pendragon" to refer to Arthur, rather than to Uther or his brother, is of much more recent vintage. In literature, one of its earliest uses to refer to Arthur is in Alfred Tennyson's poem Lancelot and Elaine, where, however, it appears as a title of Arthur rather than as a surname, following contemporary speculation that "pendragon" had been a term for an ancient British war-chief.
The term "Pen Dragon" also is a Fennian (Ephraimite) form meaning, Pen "child(ren) of" Ap "Son of" combined with "Dargon" or "Dragon" the symbol of the Tuatha de Dannon (Danites) who were warriors that married the Irish widows, hence the Gaullo term Welch (Ue Lach) or "woman of a warrior."