A poem by Cynddelw Brydydd Mawr notes a victory for Llywelyn ab Iorwerth (d.1240) in his struggle with his uncle, Daffydd ab Owain Gwynedd, in 1194 (Insley):

Cadwent pan gyrchaist, gyrch union --- sengi,
Sangei bawb ei alon;
Parthred gwyth mal Gwaith Faddon,
Porthathwy, myrdd ofwy Mon (Jones and Owen, 241, line 20)

Translation:
When you took to the battlefield, [as one] pursuing a direct course,
Everyone trampled upon their enemies;
A region full of frenzy, like the battle of Badon
Was the [Battle] of Porthaethwy, a tragedy [for] a huge army [in] Anglesey. (Jones and Owen, 241, line 20).

Porthaethwy is the earlier name for the settlement of Menai Bridge. The conflict is also noted in two poems by Prydydd y Moch (Jones, 203, line 24 and 214, line 39), but is not present in any of the chronicles.

RCAHMW (Battlefields Inventory), Feb 2017

Bibliography
Charles Insley, `The Wilderness Years of Llywelyn the Great?, Thirteenth Century England IX (2001), 163-73.
Jones, Nerys Ann and Owen, Ann Parry (gol), Gwaith Cynddelw Mawr II (Caerdydd, Gwasg Prifysgol Cymru, 1995).
Jones, Elin M. (gol.) Gwaith Llywarch ap Llewelyn `Prydyddy Moch? (Caedydd, Gwasg Prifysgol Cymru, 1991).