Facial muscles

Hyperlink
Nice diagram of the facial muscle structure. Very important to have a good grasp of this because it determines how the topology should be arranged to allow for realistic deformations and facial expressions. Good topology is always defined by the lines proximity to muscular flow. If the character is an animal then the topology should follow its biological makeup, unless it is anthropomorphised then its down to the artist discretion to create a blend.

Anthropomorphism
On the subject of Anthropomorphism, here's a series of videos with drawing techniques for drawing animals.