The White Chicken that was soul forged to Casper is found in the Bellegar Shrine south of Casper himself. Once you find it you can either kill it, which in turn will kill Casper because of the soul forgery, or leave it alone. Unfortunately, this quest was never properly implemented; there is no way to help Casper. You can neither bring the chicken to him nor can you remove the soul forging. However, if you are playing the Xbox 360 version of the game, you can kill the chicken, murdering Casper in the process, and receive an Achievement.
Casper was made in likeness of a person who represented a different publisher than Larian Studios ended up with. When pitching their idea, this person reacted by saying that his company would't be interested because Belgian companies are known to be bad at in-game art. Because he chickened out, they made a character who looked exactly like him and gave him a few chicken-like characteristics. When you kill the White Chicken, Talana will react by saying something like "I can't believe you went through all that trouble just to off that raving lunatic". This could possibly show their opinion about 'Casper', which would be a babbling fool.
It is probably just an Easter Egg and an inside joke. This could be the reason that you can't save Casper.