Old English
Beowulf, who is featured in one of the most important texts written in Old English, may very well have been real. The epic details what people Beowulf belonged to (the Geats, who resided in modern Götaland), and IIRC, battles which have taken place according to historians (particularly between the Geats and the Swedes).
Most intriguing to me are the facts that the location of Beowulf’s burial mound is included in the epic – and that there is what looks like a hill at that location in modern Sweden that has never been excavated.