Extraordinary Claims Require Extraordinary Evidence

[Esquilax]

My assumption would be a horse (neither zebras nor unicorns are common in this area).  Beyond that, I would look to others for a more detailed evaluation. (is there even a difference between horse, zebra and unicorn hoof prints?) I would look at if they are knowledgeable on the subject, are there contrary views to be accounted for, and do they site and base their claim on the evidence.

The minimum standard of evidence would be sufficient to demonstrate the claim, and rule out the others.  I'm doubtful that your hoof prints would suffice.

The point is that, were somebody to be with you and claim that a horse or zebra made those tracks, you would not need to first determine whether horses or zebras are possible. It's not an extraordinary claim because it is, itself, supported in some measure. With zebras there would be an additional set of criteria that would need to be fulfilled (are there zebras in the area? If not usually, how did one get here? etc) because that claim is slightly more extraordinary than the baseline experience of your standard western person.

When it comes to unicorns though, now there's a lot more work to do, because the existence of unicorns is not a given, nor are a series of other questions about their characteristics that would need to be asked before we could take the claim that they made hoof prints in this specific area of the world seriously. It is a claim for which none of the basic premises have been demonstrated, making it extraordinary in that it currently contravenes our understanding of the world, and you'd have extra legwork to do getting the claim demonstrated. Hell, even if we did literally find a horse with a horn making those tracks there would be additional investigation needed, both in terms of what was meant by "unicorn" in the original claim, and in terms of the nature of the creature, because they still might not line up right.

Like it or not, there is additional necessary evidence required for extraordinary claims versus mundane ones, because you can't just skip the baseline demonstration of possibility to get to a given conclusion, and you also can't just take a phenomenon as evidence of its cause: you would actually need to demonstrate that any given supernatural claim comes from an actual supernatural cause, and if you had evidence of that it would be extraordinary indeed, and would certainly require more work than regular evidence would.