The term Robber Knight is actually not mediaval, but from teh late 18th century. But if it had been in use in 1399, Hartmut the Younger of Kronberg would have fit the bill well. From the Tannenburg castle in the Odenwald forest he terrorized the surrounding lands and disturbed the trade along the Alte Bergstraße road. The authorities organized a punitive expedition ( euphemistically named the “Landfriedensheer” – the “Public Peace Army”). WIth heavy-calibre firepower, they smoked out the robber knights.
Spieltrieb Frankfurt presents medieval battles at the gates of Burg Tannenberg, using Impetus rules.