One example of a former National Park is Platt National Park in Oklahoma, established in 1906. Its National Park designation was repealed in 1976
and the land was incorporated into Chickasaw National Recreation Area
The size of Platt National Park was only 842 acres (about 1.3 sq. mi.), so its small size was a likely reason why its National Park designation was repealed and its land incorporated into a larger National Recreation Area.
gives another example, Sullys Hill National Park in North Dakota, established in 1904 by President Theodore Roosevelt. This land is now managed by the U.S. Fish and Wildlife Service.