Cousins played a total of 35 games for the Hawks since making his debut against Sydney in Round 10, 2017.
The 23-year-old featured for the Hawks on 10 occasions this year, his fifth season at Hawthorn.
Brooksby and Hartley both joined the Hawks last season after having previously played for the Eagles and Bombers respectively.
Hartley played just the five games in the brown and gold, three of which coming this season, while Brooksby managed just the one game last year.
Greaves made his debut for the club in late into the 2020 season, and showed plenty of promise in defence for the Hawks.
Adding a further seven games to his career, Greaves departs the Hawks after 10 appearances under coach Alastair Clarkson.
Pepper joined the Hawks in 2019 via the club's Next Generation Academy, however did not play a game at AFL level.
“This is always a tough time of year. Each and every one of these men have made a significant contribution to our club,” Hawks football boss Rob McCartney said, via the club's announcement.
“The past two years have been particularly challenging for all of the players and clubs across the league. The commitment of these players during that time is something that should absolutely be commended.”
“We wish them all the best on whatever they do with the next stage of their careers and lives - they will always be part of the broader Hawthorn family.”