Lancelot does not appear in the earliest existing versions of the Arthurian legends. Geoffrey of Monmouth for example doesn't mention Lancelot. Geoffrey of Monmouth a Welsh cleric gives the earliest story of Arthur's life in Historia Regum Britanniae (History of the Kings of Britain) in 1135 AD. Although Geoffrey of Monmouth is not the earliest source that mentions Arthur, it is the first to identify him as a high king from Britain's past.
Chretien de Troyes was a French poet, and he was the first great exponent of the romanticisation of Arthurian legend. He appears to be the first writer to mention Lancelot. His narrative romances, composed c.1170–c.1185 in octosyllabic rhymed couplets, and they include Érec et Énide; Cligès; Lancelot, le chevalier de la charette; Yvain, le chevalier au lion; and Perceval, le conte del Graal.
The name Lancelot does not appear to be Celtic, though attempts have been made to find a possible Celtic origin that might have been corrupted into Lancelot (such as "Lance ap Lot," meaning Lance, son of Lot). However, the name "Lance" was not known at that time. Roger Sherman Loomis has postulated that Llenlleawg, an Irish warrior of Arthur's who appears in Welsh legends, may have been the origin for the French Sir Lancelot, and Norma Goodrich has postulated that the name comes from "L'Anguselus", referring to the Scottish king Anguselus mentioned in Geoffrey of Monmouth, but these theories have never really gained much credence..
Sir Lancelot, knight of the Round Table