I think that we need to be a little more cautious. If a griffin were found and proven to be beyond any reasonable explanation in terms of evolutionary theory, then we could certainly say that the griffin had not evolved. I don't think, however, that a singular anomalous species would cast much doubt on the rest of evolutionary theory, or the explanations derived from it - a little, but not much. That is not to say that our understanding of how the griffin came to be might not cast such doubt, but concluding that just from the existence of griffins would be jumping the gun a bit.
The most we could say, given griffins, is that whatever produced griffins might have had a hand in the evolution of other species, but we would need evidence to say that it did, and the anomalous nature of the griffin would be a reason for suspecting that it did not. Given the evidence that we actually have I think that we would stick to current explanations until more evidence came in.
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