It is hypothesised by creationists that evolution cannot be true without the existence of crocoducks all around. This is an example of a Straw man fallacy. The real Theory of evolution never suggests Crocoducks but Creationists are discouraged from studying real evolution so they believe what their preachers imagine is evolution.
Richard Dawkins on the CrocoduckEdit
Why doesn't the fossil record contain a fronkey?' Well, of course, monkeys are not descended from frogs. No sane evolutionist ever said they were, or that ducks are descended from crocodiles or vice versa. Monkeys and frogs share an ancestor, which certainly looked nothing like a frog and nothing like a monkey. Maybe it looked a bit like a salamander, and we do indeed have salamander-like fossils dating from the right time. But that is not the point. Every one of the millions of species of animals shares an ancestor with every other one. If your understanding of evolution is so warped that you think we should expect to see a fronkey and a crocoduck, you should also wax sarcastic about the absence of a doggypotamus and an elephanzee. Indeed, why limit yourself to mammals? Why not a kangaroach (intermediate between kangaroo and cockroach), or an octopard (intermediate between octopus and leopard)? There's an inﬁnite number of animal names you can string together in that way.
Of course hippopotamuses are not descended from dogs, or vice versa. Chimpanzees are not descended from elephants or vice versa, just as monkeys are not descended from frogs. No modern species is descended from any other modern species (if we leave out very recent splits). Just as you can ﬁnd fossils that approximate to the common ancestor of a frog and a monkey, so you can ﬁnd fossils that approximate to the common ancestor of elephants and chimpanzees. (Richard Dawkins) 
Richard Dawkins said further,
A long time ago, there was a common ancestor between crocodiles and ducks which looked like a lizard of some sort. 
- ↑ Kirk Cameron's "Crocoduck" Came Back To Bite Him In The Ass
- ↑ Dawkins Ponders the Crocoduck in New Evolution Book: Interview