Szlachta
Mieszko I of Poland (c. 935 – 25 May 992) established an elite knightly retinue from within his army, which he depended upon for success in uniting the Lekhitic tribes and preserving the unity of his state. Documented proof exists of Mieszko I's successors utilizing such a retinue, as well.

Q: Who also established this retinue as well?
Mieszko I's successors