From Blaseball Wiki

Rhison (cerototherion simon cotton) are a human-created hybrid creature believed to only exist in the part of Yellowstone National Park located in the American state of Idaho. Coincidentally, this area is also called the Zone of Death due to a loophole created by the Sixth Amendment to the United States Constitution. Since federal crimes committed in the Zone cannot be tried, it has long been rumored to be the source of the location of federal labs experimenting on animals.