Stiffness in creature 3D models is fixed by refining rigging (adjusting bone hierarchy/weight painting for even deformation) and adding secondary motion (soft body simulations or blend shapes for organic details).
Rigging tweaks ensure joints move naturally—uneven weight painting causes "popping" when the model moves, so use your 3D software’s weight paint tool to balance how bones affect the mesh; this fixes basic stiffness fast.
Secondary motion adds subtle, lifelike movements (e.g., jiggling ears, muscle flex) that reduce rigidity. Blend shapes smooth facial/body transitions, while soft body simulations handle floppy parts like tails or skin.