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

dinosaurs mating
 n.

    Said to occur when yet another big iron merger or buyout occurs;
    originally reflected a perception by hackers that these signal another
    stage in the long, slow dying of the mainframe industry. In the mainframe
    industry's glory days of the 1960s, it was ?IBM and the Seven Dwarfs?:
    Burroughs, Control Data, General Electric, Honeywell, NCR, RCA, and Univac.
    RCA and GE sold out early, and it was ?IBM and the Bunch? (Burroughs,
    Univac, NCR, Control Data, and Honeywell) for a while. Honeywell was bought
    out by Bull; Burroughs merged with Univac to form Unisys (in 1984 ? this
    was when the phrase dinosaurs mating was coined); and in 1991 AT&T absorbed
    NCR (but spat it back out a few years later). Control Data still exists but
    is no longer in the mainframe business. In similar wave of dinosaur-matings
    as the PC business began to consolidate after 1995, Digital Equipment was
    bought by Compaq which was bought by Hewlett-Packard. More such
    earth-shaking unions of doomed giants seem inevitable.


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

dinosaurs mating

    The activity said to occur when yet another big
   iron merger or buy-out occurs; reflects a perception by
   hackers that these signal another stage in the long, slow
   dying of the mainframe industry.  Also described as
   "elephants mating": lots of noise and action at a high level,
   with an eventual outcome in the somewhat distant future.

   In its glory days of the 1960s, it was "IBM and the Seven
   Dwarves": Burroughs, Control Data, General Electric,
   Honeywell, NCR, RCA, and Univac.  Early on, RCA sold
   out to Univac and GE also sold out, and it was "IBM and the
   BUNCH" (an acronym for Burroughs, Univac, NCR, Control Data,
   and Honeywell) for a while.  Honeywell was bought out by Bull.

   Univac in turn merged with Sperry to form Sperry/Univac,
   which was later merged (although the employees of Sperry
   called it a hostile takeover) with Burroughs to form Unisys
   in 1986 (this was when the phrase "dinosaurs mating" was
   coined).  In 1991 AT&T absorbed NCR, only to spit it out
   again in 1996.  Unisys bought Convergent Technologies in
   1988 and later others.

   More such earth-shaking unions of doomed giants seem
   inevitable.

   [More dates?]

   [Jargon File]

   (1998-07-10)