Diplocaulus is an extinct and unusual amphibian, with a distinctive boomerang shaped head.  It really IS extinct, although a Japanese sculptor's image caused a big stir a few years ago.  This image made the email rounds, and just like pretty much every picture that makes the email rounds, it was (sadly) fake.  

A Japanese sculptor created an exacting replica of a small diplocaulus, then set it in a pan of water on his lawn and snapped a picture.  This proved to be sheer genius, even if he didn't intend it that way, because the naturalistic setting (as if he had just picked it up) and his wonderful work on the replica convinced thousands of people that it was real.

(I haven't been able to turn up any more information beyond "Japanese sculptor."  I hope he was able to get some good commissions off the strength of his diplocaulus sculpture!  Dude obviously has some serious chops.  If anyone knows more about the sculptor behind this image, please let me know!)

No one has yet found existing populations of Diplocaulus, although similar lizards and amphibians once thought to be extinct have turned up from time to time.  In 2000, a species of giant clumsy lizards was rediscovered in the Canary Islands (as the article points out, it "should be pretty hard to miss," but that doesn't mean it didn't take hundreds of years before it was properly described to the scientific community).  And last summer a group of legless lizards thought to be extinct were discovered in Australia's Northern Territories.

Diplocaulus was a very large salamander, being a long, legged amphibian with a lengthy tail.  They ran to about three feet in length - substantially longer than the one in the hoax photo.  Diplocaulus had relatively weak legs and a short tail, and scientists believe it swam with an up-and-down motion like a dolphin or a whale, using its boomerang head to help it swim.  Sort of like a whale with flukes on its head!

This is particularly interesting from an evolutionary perspective to compare it with the hammerhead shark.  Although both animals' heads are shaped about the same, the hammerhead's head gives it a greater sensory perception area, acting as a big scanning device for the ocean floor.  It also gives them a better, 360 degree view of their surroundings.  

By comparison, Diplocaulus had its eyes positioned right in the center (where they usually would be on a salamander).  The unwieldy head may have helped prevent predation, making it difficult for it to be swallowed.  But for the most part, what we see in the extinction of Diplocaulus is an inefficient, energy-intensive physical form not giving the long term payoff to the species to justify its usage.

Unfortunately, even regular old amphibians like bullfrogs are having a hard time these days.  Because amphibians breathe through their permeable skin, they absorb a lot more chemical contamination from their environment compared to other animals.  These are difficult times for amphibians, and hundreds of species are facing a real threat of extinction.