Early Celtic, Pict, Britain, Cambrian, etc. genealogy has never produced one "truth" on which everyone can agree. However, whether legend translates into truth might not make any difference when it comes to name origination. Names originate from legends and myth, just as easily as from agreed upon "truths". Although the early genealogies can be suspect, we will provide them anyway, because they are not necessarily wrong, and they are interesting.
Coel Hen Godhebog was a key figure in the early genealogies. He was descended from Joseph of Arimathea and was, himself, the ancestor of many princes and several saints, including St. Kentigern. His seat of power was at carlisle and he was born around 380 C.E. One descendant, in about 560 C.E. was Llwyarch Hen, known as "the Bard". He wrote poems and built fortresses. none of which has survived to today. However, two, Castle Lywar and Caer Laurie in the Lothians, survive in the stories of the area.
This warrior and lyre carrying bard had two sons, one of which, Llwyarch-Ogg, settled down on the north shore of the Solway, within a region termed Carbantorigum by the historian Ptolemy. There, in the early 7th Century, originated the greatest of the Nithsdale fortresses, Caer-Llwyarch-Ogg - the fort of Llwyarch-Ogg - named after himself. Over the years, this has evolved into Caerlaverock.