Drustwrn Hayarn is listed as a court member in Culhwch.
Drustwrn must be a scribal error of the name Drystan, which derives from Drustanos, the early Celtic version of the name. There are several inscriptions from Cornwall, Gwynedd and Brittany, but the name have survived in the Pictish language.
Drystan isn't a common Welsh name, although it is well connected with Drystan mab Tallwch - the Arthurian Tristan (Tristram).
Drustanus | The Legend of King Arthur
Picts | The Legend of King Arthur