Character model

Hello, Can anyone tell me how to change character´s model? To be a cow , chicken , etc...
I only can do this, but can make it permanent, and only in single player
http://imageshack.us/photo/my-images/543/pjhm.png/

PD: Ignore attachment

Nope, sounds awesome though

That is possible, look http://i.imgur.com/ZuCYhZu.jpg