Once a Prisoner has been returned to the Vault, there is no way to earn him in-game except for trading (more on that in a bit). The Behemoth does not have any control over awarding Prisoners to individuals. This means if you've missed one, accidentally traded one, or your Prisoner did not save, we cannot re-award him or her to you. :(
However! The eternally kind players of BattleBlock Theater are often willing to trade Prisoners for gems, balls of yarn, or other Prisoners (!). Try posting your proposed trade to the Gift Shop section of our forum.
Many of our special Prisoners also come back after being locked in the vault for some time. For example, the Behemoth Chicken was first released in early April, and he came back again in late May for our 10th Anniversary!
Keep an eye on our DevBlog for any announcements regarding returning Prisoners:
Happy Prisoner collectin'!