Horses can be rather tricky, I had to change the Model and Rig a few times to get the shoulder working right in animation.   Horse modeled in Zbrush, Rigged* and Animated in Softimage.
*GEAR by Jeremie Passerin was used.
Horse base mesh, Ztool, OBJ and Softimage rigged scene available for free download at BadKing
Also featured in 3D Artist magazine facebook page.