- MSFT and the PC OS market in the 1990s
- CATV operators in the 1970s
- AT&T in the first third of the 20th cent.
- Electric utilities (at first, GE and Westinghouse) over the same period
- The German chemical industry (again, over roughly the same period)
The Econ-101 explanation is that a competent first mover in an industry that delivers an especially high return to scale can cut prices fast enough that no new competitor in the market could ever recoup the cost of entry. So nobody bothers, and the monopoly is free to set prices until the government steps in to bring down the price or nationalize the firm.
In other words, most of the entities we traditionally associate with the term "monopoly" arise naturally and then get special legal status to recapture some of the value they generate.