The Cottonian Chronicle for the year 1088 notes:

Res filius teudur de regno suo expellitur a filiis bledit id est madauc . cadugaun . et ryrid . Ipse uero yberniam adiit . et classe accepta reuertitur in britanniam . Bellum penlethereu geritur in quo duo filii bledid id est madauc et ririd ceciderunt et Res uictor fuit (Gough-Cooper, c409.1?2).

Translation: Rhys ap Tewdwr was expelled from his kingdom by the sons of Bleddyn, that is Madog, Cadwgan and Rhiryd. However Rhys went to Ireland and returned with a fleet to Britain. The battle of Penletheru was waged, in which the two sons of Bleddyn, that is Madog and Rhiryd, were killed and Rhys was victorious (Remfry, 211).

The Brutiau drop the Pen- from the name and provide the name in several variants, Llech-y Kreu, Llychcrei and Llechryt. The Melville Richards Archive provides two possibilities for the original form of the name: Penllethr in Ceredigion (SN 5927 6283) and Penllethrychen, now known as Llettyyrychen-Fawr in Carmarthenshire (SN 459 017). The Carmarthenshire site is near the coast and is perhaps best suited for a battle involving an army that arrived as part of a fleet. It is not possible to confirm the location of the battle.

RCAHMW (Battlefields Inventory), Dec 2016

Bibliography
Gough-Cooper, Henry (ed.) The Cottonian Chronicle: Annales Cambriae, The C Text from London, British Library, Cotton MS Domitian A. i, ff. 138r?155r, online edition.
Remfry, Paul M. Annales Cambriae: A Translation of Harleian 3859: PRO E. 164/1: Cottonian Domitian, A1: Exeter Cathedral Library MS.3514 and MS Exchequer DB Neath, PRO E. 164/1 (Castle Studies Research, 2007).