The premise of the question is flawed. Most of them are nothing but imaginations.

We don’t know for every one for certain, but we have plausable explanations or backgrounds for nearly every cryptid and mythological creature. Usually, the explanation is either a diseased animal, misidentified bones, or someone looking to make money.

Yes.

Either they’ve been identified or it’s an animal with mange. For fun, go compare drawings of the Dover Demon with featherless owls, for instance.

Don’t go searching for hairless bears, though. They’re nightmare fuel.