Historical clue formulations

• Like animals in a 42-Across — 2013-05-13
• Like animals in a zoo — 2008-10-27
• Not free — 2007-06-24
• Like zoo animals — 2004-04-13
• Behind bars — 1998-12-15
• Pent up — 1997-02-03
• Pent up — 1994-10-30
• Incarcerated — 1994-03-20