I knew there was such a law in Washington state; either the state or a county. One of the first links that popped up when I Googled this was the following post from 2009 on the Cryptomundo site. There is such a law in Skamania, WA -- an ordinance prohibiting the "...premeditated, wilful and wanton slaying ..." of a Sasquatch, which is a felony under that law.
Skamania County Ordinance
Ordinance No. 69-01
Be it hereby ordained by the Board of County Commissioners of Skamania County:
Whereas, there is evidence to indicate the possible existence in Skamania County of a nocturnal primate mammal variously described as an ape-like creature or a sub-species of Homo Sapiens; and
Whereas, both legend and purported recent sightings and spoor support this possibility, and
Whereas, this creature is generally and commonly known as a “Sasquatch”, “Yeti”, “Bigfoot”, or “Giant Hairy ape”, and has resulted in an influx of scientific investigators as well as casual hunters, many armed with lethal weapons, and
Whereas, the absence of specific laws covering the taking of specimens encourages laxity in the use of firearms and other deadly devices and poses a clear and present threat to the safety and well-being of persons living or traveling within the boundaries of Skamania County as well as to the creatures themselves,
Therefore be it resolved that any premeditated, wilful and wanton slaying of such creature shall be deemed a felony punishable by a fine not to exceed Ten Thousand Dollars ($10,000) and/or imprisonment in the county jail for a period not to exceed Five (5) years.
Be it further resolved that the situation existing constitutes an emergency and as such this ordinance is effective immediately.
ADOPTED this 1st day of April, 1969.
The above ordinance was partially repealed and amended in 1984 by Ordinance 1984-2:
The ordinance was amended to make the crime a gross misdemeanor, punishable by up to one year in the county jail and/or a $1000 fine.
The new ordinance also created a million-acre refuge within the County.
Board of Commissioners of Skamania County
Some comments left at that post contain further information about laws, endangered animals, the feds, other protected cryptids, etc.