Geryon (also Geryones or Geryoneus) was the son of Chrysaor and Callirrhoë, and he was a three-headed monster with rich flocks of cattle. Robert Graves suggests geranos, 'crane' as a possible origin of his name, but this has no relevance at all to the monster.
Graves also proposes tricephalon, since Geryon was three-headed (tri- and cephale, 'head'), but this also seems too far removed from the name. Perhaps 'speaker', 'shouter', is more realistic, from geryo, 'to speak', 'utter', 'cry'. He would certainly have shouted at anyone trying to make off with his fine oxen, and fighting invariably involved war-cries.