Coming on the heels of the great bigfoot hoax, it seems mythical creatures are the flavor of the moment. So let's be entirely clear: cryptozoology is utter bollocks. While there is absolutely no doubt that there are numerous undiscovered species, the chances that the classic cryptids - Bigfoots, the various lake monsters, griffins, yetis, unicorns, etc. - exist is vanishingly small. (It's not impossible, certainly, but enormously unlikely). I will explain presently why this is the case, but for now consider the following. The minimum viable population of a large (and thus likely k-selected) animal is hundreds or thousands of individuals. With billions of people running around equipped with many millions of cameras, it's nearly inconceivable that no compelling evidence would exist if there really were thousands of individuals of some large undiscovered species. (And, if there are large cryptids they must have evolved, so where is the fossil evidence?). Moreover, the pattern-seeking human mind seems especially prone to inventing lake monsters: Wikipedia's "List of Reported Lake Monsters" is huge, including 20+ 'species' from Sweden. Even the most ardent cryptozoologist has to admit that the chances of all these stories being true is infinitesimal, which means that even true believers have to invoke the normal skeptical explanations of misidentification, hoaxing, false memories, and the general unreliability of eyewitness testimony. But, given the lack compelling evidence (like high-resolution, clearly unhoaxed video or a live specimen or a dead body), it's unclear why any of the stories ought to be taken seriously. That is, the skeptical explanations of cryptids is a bit like Daniel Dennett's universal acid: once invoked, they eat through all the purported cases.
Carl Sagan's quote "extraordinary claims require extraordinary evidence" is justly famous. But too few skeptics realize that the principle behind Sagan's line goes back to the great Scottish Enlightenment philosopher David Hume. (Indeed, it might go back even further), In the chapter "Of Miracles" in An Enquiry Concerning Human Understanding, Hume writes:
The plain consequence [of the preceding argument] is... ‘that no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavors to establish; and even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior.’ When anyone tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle. If the falsehood of his testimony would be more miraculous, than the event which he relates; then, and not till then, can he pretend to command my belief or opinion.While Hume frames his discussion in terms of miracles, they're not crucial to the argument; the principle is generalizable and even formalizeable. Basically, Hume argues that whenever we're confronted with some body of evidence, call it E, for the truth of some proposition, P, we must weigh the evidence against the probability, given everything else we know, that P is true. So let's apply this logic to the Storsjöodjuret case. The point of the previous paragraph was to establish that, in this case, the prior probability of P being true is extremely low. That is, there is only a very small chance, given everything else we know, that there are previously unknown 6-meter serpentine monsters with dog-like heads in a particular Swedish lake. And how about the evidence? Well, we have a low-resolution, indistinct video of a snake-like thing and some anecdotes. So what's the greater 'miracle'? That hundreds or thousands of huge snake-like creatures with no known ancestors live in a Swedish lake and happen never to have been filmed clearly, caught or washed up on shore? Or that the video is of something else, that well-documented human biases deceived the eyewitnesses, and that the local people (and government) are telling tall-tales to attract tourists?
Make up your own mind, but I'm with Hume on this one.