Check out the "Source Creature Rigging" video tutorial on design3.
Working in Softimage, you’ll start with the basics of bones, joints, and hierarchies as you set up an animation rig for a biped skeleton character. Then learn how to create a shadow rig and envelope the character model to it. Next, you’ll see the character guides and rigs that are already included in Softimage and learn how to modify them to fit your own needs. Finally, you’ll go through the entire process of creating a skeletal rig for a quadruped dragon character, ready to export to the Source engine.