They dont exist. Sells lots of books to the uninformed.
There are many examples of animals were thought to be mythical, a hoax, or extinct which were later found to exit. Gorillas are one. The platypus was thought to be a hoax initially. It does look improbable. The coelacanth was a fish thought to be extinct 64 million years ago. Apparently the coelacanths were unaware they were extinct as they had been breeding all that time. The Tasmanian tiger is supposed to be extinct, but there have been recent sightings.
Carcasses/ hair/ DNA usually came back. All we have is 8/16mm movies. Wake me when they get something viable.