Kobetsu/oson existed from the ancient times when Emperor was called Okimi (great king), and were created by giving family names to branch Imperial families whose lineage to the Emperor became remote or to children whose mothers were mistresses or concubines of the Emperor in order to prevent financial troubles and succession races.
皇別・王孫は天皇が大王であった古代から存在し、財政や後継者争いの防止の観点から現天皇と血筋が遠くなった傍流の皇族や、天皇の子供でも側室やめかけの子供であるものに苗字を与える形で誕生した。