Hi AlphaOmegaKid,
The hoof is an evolution of the third toe (I found out on Wikipedia, I'm no horse evolution expert) - so it's a modification of an existing feature.
If a pair of cats were killed and buried, the chances of them being fossilised and found in a million years is probably 0.00000000001%. Or less. But to answer your question, finding a single six toed cat- like creature would I suspect be treated as an anomaly /mutant rather than a new species, because mammals generally have 5 digits.
To be honest it doesn't matter that much which creatures in the fossil record could have interbred or not. If evolution is true then breeding across 'species boundaries' must have happened millions of times.
You're right we can't actually prove that one creature descended from another if we find their fossilized remains. But as far as I'm concerned finding a sequence of fossils that tell a story, in dated rocks, is pretty good evidence. I'm sure you disagree with that but it convinces me.
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