***Warning*** This is kinda gross but extremely fascinating.
Cymothoa exigua extracts blood through the claws on its front, causing the tongue to atrophy from lack of blood. The parasite then replaces the fish's tongue by attaching its own body to the muscles of the tongue stub. The fish is able to use the parasite just like a normal tongue. It appears that the parasite does not cause any other damage to the host fish. Once C. exigua replaces the tongue, some feed on the host's blood and many others feed on fish mucus. This is the only known case of a parasite functionally replacing a host organ. There are many species of Cymothoa, but only C. exigua is known to consume and replace its host's tongue.
Proving once again that truth is stranger than fiction...
Both of the linked sources have more detail on this unique creature. Well worth the read if you'd like more information. The various lengths that some creatures go to just to survive and procreate is an endless source of amazement to me and this ranks right up near the top of my list of strange and unusual!
So what odd creatures are on your list?