Lancelot was confused with Peredur, and Guinevere with the lady of
whom Peredur was in quest. The Elaine who becomes by Lancelot the
mother of Galahad "was Lancelot's rightful consort, as one recognises
in her name that of Elen, the Empress, whom the story of Peredur"
(Lancelot, by the confusion) "gives that hero to wife." The second
Elaine, the maid of Astolat, is another refraction from the original
Elen. As to the Grail, it may be a Christianised rendering of one or
another of the magical and mystic caldrons of Welsh or Irish legend.
There is even an apparent Celtic source of the mysterious fisher king
of the Grail romance. {12}

A sketch of the evolution of the Arthurian legends might run thus:-

Sixth to eighth century, growth of myth about an Arthur, real, or
supposed to be real.

Tenth century, the Duchies of Normandy and Brittany are in close
relations; by the eleventh century Normans know Celtic Arthurian
stories.

After, 1066, Normans in contact with the Celtic peoples of this
island are in touch with the Arthur tales.

1130-1145, works on Arthurian matter by Geoffrey of Monmouth.

1155, Wace's French translation of Geoffrey.

1150-1182, Chretien de Troyes writes poems on Arthurian topics.

French prose romances on Arthur, from, say, 1180 to 1250. Those