Dafydd Penmachno Goch or Dafydd ap Dafydd ap Gruffudd is said in some genealogical sources to be the illegitimate and only surviving son of Dafydd III the last free Welsh Prince of Wales (December 1282 – June 1283).
Dafydd ap Gruffudd was married to Elizabeth Ferrers and is known to have had two legitimate sons (Llywelyn and Owain) and one legitimate daughter, Gwladys. The two sons died imprisoned at Bristol Castle while Gwladys spent her days as a prisoner in all but name at the Gilbertine Convent at Sixhills in Lincolnshire. Dafydd is also reported to have had seven illegitimate daughters. Their fates are unknown, but Edward I appears to have considered sending them to Gilbertine convents as he did with Gwladys.
Little is known of the life of Dafydd Goch. It could be presumed from his epithet Goch – meaning "red" in Welsh – that he was either born with red hair or that his life was associated with blood or violence. His mother is in genealogical sources said to be Tangwystl ferch Owain Fflam, an unknown Welsh woman. He took a wife called Angharad ferch Heilyn.
His son, Gruffudd ap Dafydd Goch, who died c. 1365, is buried at the church of Betws-y-Coed where there is a stone effigy in his honour that details his genealogy. He also had a daughter, Gwenllian, who married Gruffudd ap Iorwerth, hereditary baron of Edeirnion and an ancestor of the Hughes of Gwerclas family (see: Powys Fadog).
Gruffudd ap Dafydd Goch is reported to have had a son called Gruffudd Fychan ap Gruffudd who had a son himself called Gruffudd Leiaf who was a poet and an englyn written by him is found in Cwrtmawr. This Gruffudd is recorded in the sources as having a son called Ieuan ap Gruffudd Leiaf who was also a poet. Some examples of his work remain in manuscript, including cywyddau and awdlau to members of the Penrhyn and Nanconwy families, vaticinatory and religious poems, a poem on Aberconwy, a satire on the Llugwy river for hindering the poet while journeying to Penrhyn, and a short bardic controversy, or ymryson, composed between the poet Guto'r Glyn and himself.
"Syr" Siôn Leiaf is recorded as being one of the sons, alongside Robert Leiaf, of Gruffudd Leiaf (ap Gruffudd Fychan ap Gruffudd ap Dafydd Goch ap Dafydd ap Gruffudd ap Llywelyn Fawr). Nothing is known of his life apart from his being a cleric and another poet. A number of his poems remain in manuscript. These include two religious poems, one being a confession and the other a poem on the vernicle, a poem in praise of Richard Kyffin, dean of Bangor, a love poem, and another to the owl. He died c. 1480.
It is possible that his descendants and those of his male relatives – if they survive – represent a direct surviving branch of the House of Aberffraw and would therefore be claimants to the long dormant throne of Gwynedd. It would also make them rivals to the claims (now extinct) made by John Wynn and the Lords of Gwydyr. However, under Welsh Law an illegitimate son must first be acknowledged by his father to have any claim of inheritance, and of this there is no known record.