Welsh, derived from "Matrona," Gaulic Celtic mother goddess
, meaning "Great Mother."
Daughter of Afallach, king of the Otherworld Avalon. In this aspect, she is identifiable with Morgan le Fay, and the Lady of the Lake. She is a Mother Goddess, in this case mother of Mabon ap Modron, supernatural hero and seasonal god, and thus identifiable with Rhiannon--both lost their sons at three days old, and were punished for it.
Based upon "Pa Gwr," she is thought to have been impregnated (gods don't really get married, though they say they do) by a hypothetical god Meldos or Mellt. There has yet to be any literature produced to prove there was a god called Meldos or Mellt.
In some Welsh texts, she is said to be the wife of Urien of Rheged, some texts say Uriens of Gorre (the latter being a fictional version of the former, who was a historical king in North Britain). As such, she is replaced by Morgan le Fay in Le Morte d'Arthur.