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Natural monopolies, by definition, arise from natural market forces, and thus do not rely on government intervention or unfair business practices to suppress competition. (That doesn't mean natural monopolies always compete fairly or never engage in regulatory capture. Only that these are not necessary for them to become monopolies.) Railroads are a commonly cited example. Today, the network effects of digital platforms are thought to give rise to monopolies (or at least high market concentration).

People who usually ask "give me an example of a monopoly not maintained by the government" are usually not asking in good faith. Many laissez faire capitalists explain away natural monopolies, or pretend they don't exist. So perhaps you expected this answer, and are already prepared to dismiss it.