The story of Henwen is found in the Welsh Triad (See Other Works) "The Three1 Powerful Swineherds of the Island of Britain"*2:

...Coll son of Collfrewy, who guarded Henwen, the sow of Dallwyr Dallben, who went about to bring forth to Penrhyn Awstin [Aust] in Cornwall, and there she went into the sea. And at Aber Tarogi [Mouth of the Troggy] in Gwent Is Coed she came to land. And Coll son of Collfrewy with his hand on her bristles wherever she went, whether by sea or by land. And in the Wheat Field in Gwent [At the foot of Mynydd Llwyd: Gray Hill] she brought forth a grain of wheat and a bee; and therefore that place is the best for wheat and bees. And from there she went to Llonion in Pembroke, and there she brought forth a grain of barley and a bee. And therefore Llonion [Lanion?] is the best place for Barley. From thence she made for the Hill of Cyferthwch ["Slope of groaning"] in Eryri [Snowdonia]; there she brought forth a wolf-cub and a young eagle. And Coll son of Collfrewy gave the eagle to Bre(r)nnach the Irishman of the North, and the wolf he gave to Me(n)waedd of...of Arllechwedd; and these were the Wolf of Me(n)waedd and the Eagle of Brennach. And from thence she went to the Black Stone in Llanfair in Arfon [Llanfair Isgaer?, around Llanfair Hall], and there she brought forth a kitten; and Coll son of Collfrewy threw that kitten into the Menai. And she was afterwards Palug's Cat.

As can be seen, the pig makes, broadly, the reverse trip to the Twrch Trwyth, possibly firming up the identification of the Wonder site with the mouth of the Troggy. Like the Twrch tale this tale is designed to explain the names of places along the route, and Bromwich suggests it includes the Tarogi because the sow was Torrawc* ("pregnant"*, from tor: "belly").

Although they don't really relate to our wonder, you might also like to read some of the lives of the British 'Dark Age' saints who lived on the coast of the Severn and which give an idea of the kind of area it was near the time the Wonder list was being written: St.Tatheus; St.Gwynllyw; St.Cadog; St.Dubricius; St.Illtud; St.Oudoceus; St Samson of Dol; St.Teilo.