A fish-dog or amphibian-bird, or whatever, a thing far off the predicted tree of common descent, would be evidence against evolution. ID, a much looser hypothesis, would still be "fulfilled," since no find ever can falsify the idea that "God could have done THAT." However, that theory doesn't really tell you anything about what to expect. Thus, it's useless. There is a strong form of literal-Genesis creation which makes very strong predictions which were falsified before the end of the 19th century. That's useless, too, because it's wrong.