Easily at a first glance you are correct. One single account of a supernatural finding is far from enough to prove it. Though, if there were a sufficient number of first hand accounts, with similar details, and reliable facts, that should be enough. Though in some cases where this is absurd, such as "big foot" or the beast of bray road, one can assume that if there is a multitude of accounts that the supernatural being can indeed be believable. I am assuming that the supernatural being is new, and has no real media connected to it. That there are NO previous accounts of it. People tend to lie about things that are "supernatural" or "unreal" claiming that they have seen it when they haven't, if multiple people see the same thing with a good amount of details, and the creature has no media on it yet and no tags, then one can say that it is somewhat plausible.