Rao Sonag was a rajput belonging to the Rathore clan. His father was Rao Sheoji and his mother hailed from the Chavda clan of rajputs.
Sonag was a brother of Rao Asthan, ruler of Kheda. The two brothers together conquered the town of Idar and Sonag settled there. Consequently, his male-line descendants are known as "Idariya Rathores."