Pen son of Nethawc = Uther Pen it is!

William Frederick Yeames, Cordelia, 1888

I've now had word from Dr. Simon Rodway of The University of Wales that he agrees with the author of the following study when it comes to the name Pen[n] son of Nethawc of the MABINOGION tale "Culhwch and Olwen."

https://openaccess.leidenuniv.nl/bitstream/handle/1887/40632/05.pdf?sequence=8

I've pasted the relevant section from the study below.  But suffice it to say that I have no reason to doubt at this point that the Pen son of Nethawc (= Nwython/Nechton/Neithon/Neidaon) who fought with Gwythyr against Gwyn for Creiddylad is the same as the Uther Pen who fought alongside Gwythyr in the MARWNAT VTHYR PEN poem. Given the sphere of Uther and Arthur's activity in the North, the only plausible father for Nethawc - geographically AND genealogically speaking - is Senyllt.  The latter is thought to be the 'Liberalis' of the Yarrow Stone.

We also need to remember that Bromwich and Thompson make the following comment on the name Penn (in the English edition of CO, published in 1992; information courtesy Will Parker via personal correspondence):

"Penn uab Nethawc: some element appears here to be lacking. Pen(n) is itself an unlikely personal name."

They say this because Penn looks to be an epithet.  And that is because, in all likelihood, the name Uther has dropped out, perhaps precisely because it was mistaken for an adjective.