If a mythical creature such as a griffin existed.....

It depends on how griffin-like the creature is. If you're talking about an animal that is clearly a lion/eagle collage, then, yeah, evolution is an unlikely explanation for it.But, if you just mean an animal with wings in addition to a normal complement of legs, that might be explainable by evolution, but it depends on the details of the system. For example, arthropods usually have one pair of appendages per body segment, but insects have wings in addition to legs on two of their body segments.However, either way it goes, there is still good evidence that evolution explains at least some of the diversity of life, so discovering something that can't be explained by evolution would only indicate that we need more mechanisms, not that our current mechanisms need to be abandoned wholesale.But, like Dr Adequate said, it would open the door to debates about whether each individual animal is best explained by evolution or by whatever mechanism explains the griffin.