If we are talking about sightings of anomalous animals, we are talking about non-reproducible events. You may find the claimed event fanciful, even impossible, but to demand proof of the claimed event is unrealistic on its face. And to claim the person is knowingly lying because they do not meet your unrealistic expectation of evidence is, to my mind, a faulty line of reasoning.
Your posts have helped me understand a certain point of view. Sticking to just sightings, the beginning and end of a certain skepticism (I won't say it is necessarily a general skeptical viewpoint) is that Bigfoot does not exist, therefore, the sighting claimant is not telling the truth, he can't be, therefore his claim is not true, therefore he is saying something untrue, therefore he is lying. Does this sketch fairly represent your view?
And speaking of "sightings," even in daily life, I find people misperceive events a lot. Somewhere above in the thread I recount how I thought I saw a giant, really giant, garden spider one time.
And I like to tell this story because it illustrates how expectations can actually change, in your mind, what you are seeing. At work, once, during lunch hour, a co-worker came walking by my desk, took a look at my up-side down hat with work gloves inside, and said, "that's a good-looking hamburger you've got there." When I said "what?," he stopped, took a second look, and turned red. He couldn't have been more than 10 ft. away when he mistook my hat and gloves for a hamburger. It was lunchtime and he had food on his mind.