Theodoric m. Gothic þiuda 'people, folk' + Old High German rih 'ruler'.
The name of various early kings of the Visigoths, kings of the Ostrogoths, and Frankish kings, a 6th C Anglo-Saxon king, a 7th C Welsh king, and a 12th C antipope.
This name gave rise to a wide variety of modern forms which are not obviously connected if the origins of the names are not known: English Derek influenced by Dutch Derrick, Low German Diderik; French T(h)ierry; German Dietrich. Often confused with Theodore.