This herb used to be called the Mad Dog Weed or Mad Dog Skullcap because it was thought to cure rabies. It can be used as a fresh herb, dried herb or tincture. It can help relieve nervous tension and excitability in dogs and cats. It is often used to help with pain recovery or trauma in these animals and unlike some other calming herbs, it does not make the pet drowsy. You can find a great source of dried Skullcap at Mountain Rose Herbs.
Here is a good article to show all the ways this herb has been used:
Scullcap is an Effective Calming Herb
A couple of good books to pick up to help you:
Herbs for Pets by Wulff-Tilford & Tilford
Mountain Medicine by Darryl Patton