狗尾续貂

idiom

to add a poor sequel to a fine work; to add an inferior ending to a superior beginning; (figurative) to ruin a masterpiece with a shoddy continuation

Originated from the Eastern Jin dynasty, where officials' hats decorated with sable tails (貂尾) later used dog tails (狗尾) when sable was scarce. Now used to criticize literary sequels or any continuation that fails to match the quality of the original.