Just imagine some of earths earliest explorers probably had the sh1t scared out of them just by seeing Earths own creatures , Imagine going thru a tropical jungle for the first time , only to hear leaves wildy rustling above you, and pairs of illuminscent human like eyes peeering down at you and a series of loud unhuman grunts and howls.
This would be Mountain gorillas in the trees
Or imagine seeing the ocean for the first time , and deciding to take a swim only to be eaten alive by a great white shark ....
Or tales of 60 foot long snakes that used to inhabit the amazon ... somebody had to see these creatures with their own eyes to make such tails ... But for mysterious creatures , I think a real life Lizard Man would scare the bejesus out of me , I think I would be fascinated more than scared of say a grey alien .... I suppose a Vampire or Werewolk would be quite frightening as well. But something like Moth Man, maybe its just the name ... I cant get myself all worked up and scared over a giant MOTH ( even though ) thats hardly the description. Interesting post, Enjoy all the answers and writeups.