Under the pretense of "States' Rights", Republicans want to abandon multiple rules and even the whole Antiquities Act of 1906 (which enables presidents to declare National Monuments, which are often the precursor to National Parks).
Their goal is to open National Parks and other publicly owned lands to private exploitation—for livestock, mining and "hunting".
Killing wild horses
A long-standing issue in the American West is wild horses and burros—experts agree that there is too many (currently just under 100,000 animals) to be sustainable for the land. Groups like the American Humane Society have developed efficient programs of neutering and other non-lethal ways of management to lower the number of horses.
Congress watched as these herds overwhelmed the land’s ability to sustain them, crowded out indigenous plant and other animal species, threatened the survival of species listed under the Endangered Species Act, invaded private and permitted public land, disturbed private property rights, and turned the sod into concrete.
[…] animal welfare organizations such as the American Society for the Prevention of Cruelty to Animals and the Humane Society of the United States recognize that the prosperity of wild horses and burros on public lands is threatened […]
Congress must enact laws permitting the BLM to dispose humanely of these animals.
Project 2020, page 528
Republicans want to kill the animals (and sell them for profit for human consumption), being misleading about that the animals cause great damage and implying that animal welfare organizations want to euthanize the wild horses. What they conveniently do not mention is that there is also about 1.5 million private cattle grazing on BLM-managed land, with little compensation. In fact, Project 2025 doesn't mention cattle at all.
Bear baiting
Another proposal is to allow bear baiting, a hunting method involving severely interfering with natural resources, endangering both people and bears.
"Revoke National Park Service and U.S. Fish and Wildlife Service rules regarding predator control and bear baiting"
Project 2025, page 531
The end of the road for National Parks
They also want to inhibit presidents from declaring new National Monuments to protect our natural and cultural treasures and preserve them for future generations:
"[…] the new Administration must seek repeal of the Antiquities Act of 1906"
Project 2025, page 532
This is a thinly-veiled attempt to prevent new National Parks or expansions to them, laying the groundwork for selling them off to the highest bidder.