The patent for Monopoly is https://patents.google.com/patent/US2026082A/en which expired in '52

There is also issues of trademark on the '-opoly' games... which was settled in '83 with https://www.americanbar.org/groups/intellectual_property_law...

> Finally, in 1983, the Supreme Court let stand an appeals court ruling that the word “Monopoly” had become generic, because purchasers associated the name with the product rather than with the source.7 Specifically, evidence showed that purchasers of Monopoly were motivated by their knowledge of the game, and not by its association with Parker Brothers. Directly as a result of this case, Congress amended § 14(c) of the Lanham Act, which addresses cancellation of a registered mark, later that year. Congress added language clarifying that a mark becomes generic only if its “primary significance . . . to the relevant public” is as the generic description of the particular goods or services, and that purchaser motivation is not used as the test.

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There are clones of Monopoly because the patent expired long ago and the trademark was lost in 83. ... Which is also why McDonalds did its Monopoly game in '87.