Call me a dumbass because I have no factual evidence or proof but my opinion is that it was around the 17th/18th centuries. The reason (lol) I think that is because in the age of reason (17th century) people became more rational due to increased opportunities for education, free thinking, literature whatever else etc and because of that mystical creatures became less frightening. Because people were able to realize these things no one had ever seen probably did not exist and so forth became that of humor, lore, points of interest instead of something to actually fear etc. In movies and books dragons are still often portrayed as bad guys (ie the hobbit, reign of fire, world of Warcraft etc)
I could be wrong, I could be way off, just a guess
