The story
From the name of the daughter of Hengist and wife of Vortigern in Geoffrey of Monmouth's 12th century pseudohistorical Latin work Historia regum Britanniae. There have been attempts to connect it to some unattested Old English or Common Brittonic name, but without any real evidence. It was revived after Walter Scott used it in his novel Ivanhoe (1819). Some places are named after Lady Rowena in the novel.
Etymology from Wiktionary, CC BY-SA 4.0.