If you want proof the Bigfoot is real - you need to produce one, be it live or dead. And make it a real one not a costume with a few guts thrown in for effect.
If you want to prove the extraterrestrial craft are real, we need to be able to walk up to it and touch it. And I mean “WE”, not hearsay stories about a crash and cover-up decades ago.
And if you want proof the Mermaids are real, my wife has to find her in my bathtub.
Next thing you’ll be asking for concrete evidence that man is causing globull warming.