In 1977, the US Department of Interior's Fish and Wildlife Service published a fascinating document asking what the government would do if Bigfoot or something like the Loch Ness Monster were to be found? The paper goes on to explain the laws and regulations in place to deal with such a discovery, and also mentions 20th century discoveries like the Komodo dragon and cryptozoology's darling, the coelacanth. From the document:
Finding a Loch Ness monster or Bigfoot is still a possibility, and the discovery would be one of the most important in modern history. As items of scientific and public interest they would surely command more attention than the moon rocks. Millions of curiosity seekers”and thou- sands of zoologists and anthropologists throughout the world would be eager to “get at” the creatures to examine, protect, capture, or just look at them....
Under U.S. Law, the Secretary of the Interior is empowered to list as threatened or endangered a species for 120 days on an emergency basis. For endangered species in the United States, the Secretary can also desig- nate habitat that is critical to their survival. No Federal agency could then authorize, fund, or carry out any activities which would adversely modify that habitat.
So long-term Federal protection of Nessie or Bigfoot would basically be a matter of following the same regulatory mechanisms already used in protecting whooping cranes and tigers.
“Under normal situations,” said Keith Schreiner, then Associate Director of the U.S. Fish and Wildlife Service. “we must know a great deal about a species before we list it. How big is the population? Does it occur anywhere else? Is the population in danger of decline? Is its habitat secure? Is the species being exploited? What is its reproductive rate? Obviously, if a Bigfoot really were found we could use emergency provisions of the Act to protect it immediately. But for the record, I seriously doubt whether such a creature really does exist.”
"Are We Ready for 'Bigfoot' or the Loch Ness Monster?" (PDF from fws.gov, via Mysterious Universe)