I don't know for sure, but Horned Vulcan was cuckolded by his wife Venus (with Mars), in a 17th century broadside ballad.
"Holy Grail' is a late addition to the Arthurian tales. It first appears as 'graal' in one of Chretein de Troyes Arthurian romances. It wasn't holy yet, and appears to have been a plate. On the 'Grail' see Graham Phillips & Martin Keatman's 'King Arthur, The True Story', 1992.