It is known as “Bedd Morris”, which Morris or Morus was a notorious robber who lived among the rocks on the summit of the hill commanding the pass; and which is the old, and was once the only, road to Newport.
This man had a little dog trained to fetch the arrows shot at unfortunate way-farers. The nuisance of this murderous individual was so great that at last the population rose in arms against him, attacked him in his mountain-cave, dragged him down to the place where the stone now stands, and there killed and buried him.
From Archaeologia Cambrensis v6, 1875, in an article called ‘On Pillar-Stones in Wales’ by E.L. Barnwell.