Lyonesse is the name of a country in Arthurian legend bordering Cornwall, the home of Tristan whose father was its king, as well as also being the site of the final battle between King Arthur and Mordred. It was said to have sunk beneath the waters. It’s also the name of an Arthurian character, Lyonesse, the sister of Lynette, in the story of Gareth and Lynette. Lyonesse does sound like a variant spelling of Lioness, the name of a female lion, though I’ve also seen it listed as being the English form of French of Léoneis or Léonois, the French form of Lodonesia which is the Latin name for Lothian, a region in Scotland. The etymology of Lothian is unknown.