Branwen uerch/verch/ferch Lyr/Llŷr (Branwen, daughter of Llŷr), generally included in the Mabinogi (Mabinogion). Fourteenth Century AD, possibly as early as Third Century BC:

‘NLW MS. Peniarth 4 (The White Book of Rhydderch, part 2) – page 10r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Pen4&page=10r

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 179v’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=179v

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 180r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=180r

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 181r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=181r

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 181v’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=181v

‘Oxford Jesus College MS. 111 (The Red Book of Hergest) – page 182r’ at Welsh Prose 1350-1425, 2007-2017.
http://www.rhyddiaithganoloesol.caerdydd.ac.uk/en/ms-page.php?ms=Jesus111&page=182r

‘Branwen, daughter of Llyr’ at Academy for Ancient Texts. Ancient texts library.
http://www.ancienttexts.org/library/celtic/ctexts/branwen.html

Parker, Will M, ed, and Sionedd Davies, tr. ‘Branwen ferch Lyr: The Second Branch of the Mabinogi (The Mabinogi of Branwen)’ at The Mabinogion and the Four Branches of the Mabinogi, 2003.
http://www.mabinogi.net/branwen.htm