Cad/Kat Goddau/God(d)eu (Battle of Trees), from Llyfr Taliessin (Book of Taliesin). early Fourteenth Century AD, originally Twelfth Century AD or earlier (possibly Tenth or Sixth Century AD, or as early as 400 BC?):

‘Kat Godeu: Llyfr Taliesin VIII’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/t08w.html

‘The Battle of the Trees: The Book of Taliesin VIII, from The Four Ancient Books of Wales’ at Academy for Ancient Texts. Ancient Texts Library.
http://www.ancienttexts.org/library/celtic/ctexts/t08.html