The Jargon File (version 4.4.7, 29 Dec 2003):

dinosaur pen
 n.

    A traditional mainframe computer room complete with raised flooring,
    special power, its own ultra-heavy-duty air conditioning, and a side order
    of Halon fire extinguishers. See boa.


The Free On-line Dictionary of Computing (30 December 2018):

dinosaur pen

   A traditional mainframe computer room complete with raised
   flooring, special power, its own ultra-heavy-duty air
   conditioning, and a side order of Halon fire extinguishers.

   See boa.

   (1995-11-17)