Disable IK on the horse's legs (Figure menu --> Use Inverse Kinematics), and try again. As I said, Walk Designer can be somewhat unpredictable with custom files. Sometimes I have to shut Poser down, then start a new session before the figure will load into Walk Designer properly. Usually, if it doesn't look right in the Walk Designer's preview window, the walk won't apply properly either, but that isn't always the case.