Creating the Skeleton
A skeleton for a quadruped is fairly easy to build. A quadruped skeleton is similar to a human skeleton, as you can see in Figure 3. The animal still has hips and shoulders connected via a spinal column, and the spine is aligned horizontally rather than vertically. Another factor to consider is that that the bones for the limbs are of different lengths than in a human. As seen in Figure 4, the best way to build the legs is to create them as IK chains, with the feet attached hierarchically.
Figure 3 A quadruped skeleton is similar to a human skeleton, but it is aligned horizontally rather than vertically.
Figure 4 The best way to build the legs is to create them as IK chains, with the feet attached hierarchically.
The center of gravity might cause you to set up the hierarchy of a quadrupedal skeleton slightly differently. Because both the front and the back legs move equally, the hierarchy can be set up with the center of the spine as the parent. Having the root of the hierarchy here will make things easier for things like bending the spine. For many four-legged animals, having the root of hierarchy at the hips also works fine.
Most four-legged mammals also have tails. A tail is easy to configure as a simple chain of linked bones connected to the hips, as shown in Figure 5. These bones are typically animated using forward kinematics, which, in turn, deform the skin of the character.
Figure 5 An animal's tail can be deformed by using a simple chain of bones.
To keep animation simple, some animators like to tie the many bones of a tail to a single set of sliders that controls the bone's rotation. This is as easy as linking the rotation of each bone to a slider. Moving one slider rotates many bones.