The Crocoduck fallacy is one made by Apologist Kirk Cameron and was refuted immediately. All it ends up being is a straw man fallacy. Even though it was refuted Kirk was found to continuously use it mostly with his partner Ray Comfort (The banana man).
The argument states that for evolution to be proved right, the sets of transitional fossils need to be completed, and according to Kirk, creatures such as the Crocoduck (Duck with a Crocodile head) need to be discovered. This fallacy doesn't need much explaining: the theory of evolution does not state that a crocodile has to become a duck or that that precisely happened for evolution to be correct, even if a duck was supposed to have evolved directly into a crocodile nothing says that a duck would have a crocodile head aside from his photoshopped picture.
The Crocoduck fallacy demonstrates Comfort's and Cameron's lack of understanding of biological evolution. Comfort and Cameron apparently believe that Darwinian evolution requires a direct, single-step ancestor between every species on the planet - there must have been not only a crocoduck, but also a croco-beaver and a beaver-duck predating the crocodile, beaver, and duck.