The man who started the Bigfoot craze, in 1958, was found to have faked some footprints, and he later worked with the man who claimed to have filmed a yeti on his first stab at making a documentary, from which the following 'lovely bunch of coconuts' still photo is taken. That film is now widely considered a hoax, especially after a Hollywood special effects man later claimed he made the suit. These stunts notably followed earlier Everest climbers' internationally publicised claims to have seen a yeti in the Himalayas.
The Loch Ness monster tales go back hundreds of years, but it wasn't until a 'reputable' man photographed the 'monster', a surgeon, someone seen then, and now, as a reliable witness due to his profession, that the international public imagination caught on, and again other versions appeared in lakes around the world. Years later the surgeon's photo was proven to have been a hoax, admitted to 60 years on.
I suspect that what happens is some historians uncover redundant mythology, some of it tribal and dating back to a time when the Sun was worshipped and some animals were considered to possess supernatural powers, and in the 20th century they staged hoaxes which carried greater credibility because they appeared to have a researched lineage with the past. Then, seeing the success of the hoax, other countries developed their own versions, in the same manner the recent 2001 Space Odyssey monoliths appearing over the New Year were copied around the world.
So there is a pattern to these things, and meanwhile one thing remains a constant - there is never any evidence.