Grand Duchy of Lithuania a political force known as the Cossacks were to starting wrestle for political power,
That took place as late as 17th century. While much earlier, already in 15th century, the Grand Duchy of Lithuania had started developing their own aristocratic elites which were Polonised Lithuanians and Ruthenians.
Main families included: Sanguszko, Czartoryski, Ostrogski, Radziwiłł, Wiśniowiecki etc etc.
en.wikipedia.org/wiki/Lithuanian_nobility#Grand_Duchy_of_Lithuania_before_the_Union_of_Lublin_(1386-1569)