Back in the day, it was "IBM and the seven dwarfs", Digital Equipment, Control Data, General Electric, RCA, Univac, Burroughs, and Honeywell. Later, Amdahl offered fully-IBM-mainframe compatible systems. Tandem's specialty was fully redundant, fault-tolerant systems.

https://it.toolbox.com/blogs/shayne-nelson/the-60s-ibm-the-s...

By the 1990s, "big iron" referred not only to mainframe (S360 descendants -- now S390 / zOS), but big Unix vendors: Sun (now Oracle), HP, Data General, SGI, etc.

Some of these still survive in some form:

https://en.wikipedia.org/wiki/Mainframe_computer#Current_mar...