I can't prove that a specific thing does not exist, no. ;-) Nor can you. We can simply have opinions about it regarding its probability of existing, correct? And those opinions need some backing up, hopefully.

After one has seen a given thing...any given thing...one tends to feel that the probability of it existing has gone way, way up, to say the least. One who has not seen it, however, simply has his old opinion to fall back on, doesn't he? And that's all he has. His old opinion is quite satisfying for him, but perhaps not for someone else. That's how it is with opinions. They're highly individual.

I have as yet met no one who has seen or who claims to have seen most of your unevidenced examples. Not one person. They're amusing! ;-) But since I've met absolutely no one who claims to have seen:

"elves or orcs"..."bears that talk to little boys and get their heads stuck in jars of honey"..."an elephant's trunk that didn't develop because a child full of insatiable curtiosity got his nose stretched by a crocodile on the banks of the great grey-green greasy Limpopo River" or "a bogey under the bed".....

And I mean, NO one I've ever known has claimed to have seen those things...because of that, I'm not very impressed at the probability of any of those things existing. Clear?

On the other hand, I am quite impressed at the probability of things existing which literally thousands or by this time hundreds of thousands of people claim to have seen, including Jimmy Carter, a former minister of the Canadian defence establishment, professional pilots, and leaders of various other countries, indeed professionals from every walk of life, including some scientists, plus many, many ordinary people I've met personally....now THAT impresses me as imbuing the idea with a great deal of probability! See what I mean.

It is not in the same category as your Elves, Orcs, Winnie-the-Pooh, and other such fanciful stuff which is found only, I think, in entertaining books of fiction....so I can only wonder why you would even bother bringing up such unlikely and flimsy examples to back an argument?