Full Description
Once, Shirion was a talented bio-engineer upon the world of Erilynne. And he had, or so he thought, achieved the ultimate goal of his profession. He had created a system which would maintain the human body, healthy and unaging. It would destroy cancers, keep the genes properly sequenced, and fight the disorders that the body brought upon itself. Methusalyzation would come to the galaxy. All he needed was a test subject. Unable to gain official approval, he chose to test it on the subject closest at hand.. himself. 
The program was set. The injections begun. And unbeknownst to him, humanity’s foe had launched it’s attack, and upon Erilynne, a portion of that attack would prove to be a powerful viral plague, one designed to attack the very essence of Earth’s life. Each engineered to near-perfection, the immortality system and the sequencing plague warped and twisted one another, beginning an eternal war over the essential information that defined the human’s body.
Soon, the battle lines were drawn. The system of immortality would be able to keep him alive, but not able to keep up with the induced mutations.  
Now, Shirion’s body constantly reshapes itself essentially randomly, over the months skin shifting to hide, to scale, to feather, to chitin, as arms, legs, wings, and tentacles sprout and return to an ever-changing torso. When encountered, he may be armed with claws, fangs, beaks, or even tool-using hands, and while frequently possessed of an animal cunning, he is never sentient.
Additional Information
While his physical, and even mental abilities, change with the rise and fall of the sun, there are a small handful of constants that remain. He always has some form of locomotion available to him. He is always able to eat meat, and usually prefers it. 
He tends to range around the ruins of the ancient bio-plants, occasionally even venturing into them, driven by reasons he no longer has names for.

Shirion is the only living remnant of mankind on Erilynne, those few who survived the war and plague having died of varying other causes. However, he is not quite perfectly immortal. While the mutability of his body will swiftly erase any wounds, and time itself has not a prayer of aging something so constantly reborn, he can be destroyed by way of massive damage, and by things such as fire and drownings. 

Erilynne, the setting: http://www.strolen.com/guild/index.php/topic,3346.0.html