The Collaborative International Dictionary of English v.0.48:
Unpen \Un*pen"\, v. t. [1st pref. un- + pen.]
To release from a pen or from confinement. "If a man unpens
another's water." --Blackstone.
[1913 Webster]
Moby Thesaurus II by Grady Ward, 1.0:
28 Moby Thesaurus words for "unpen":
loose, loosen, unbar, unbind, unbolt, unbridle, unbuckle, uncage,
unchain, unfetter, ungag, unhandcuff, unhobble, unlash, unlatch,
unleash, unlock, unloose, unloosen, unmanacle, unmuzzle, unpinion,
unshackle, unstrap, untether, untie, untruss, unyoke