Allpets.com
comprehensive pet magazine and encyclopedia. In-depth info on petcare, health, breeds, showing, training. Messageboards. For pet enthusiasts and dedicated fanciers.

AVMA Care for Pets
Authoritative American Veterinary site with lots of useful pet health links.

Merck Veterinary Manual Online
One of the most comprehensive electronic references for animal care information. Includes over 12,000 indexed topics and over 1200 illustrations . Search by topic, species, specialty, and disease.

PetEducation
Advice on caring for a variety of pets.

VetCentric.com
The "encyclopedia" is a quick locator for animal healthcare topics

Cats

Cat Anatomy
Basic cat anatomy with the organ systems courtesy of Washington State University.

Dogs

Dog Anatomy
An informative site with illustrations, which also discusses variation between breeds and the factors that influence a dog's anatomy.

Dog Breeds A-Z
Courtesy of Dogs Central

Horses

Horse Breeds of the World
Search more than 85 different breeds of horses worldwide by breed or by continent. From the International Museum of the Horse, the site provides pictures, history & origin of the breed, breed characteristics, champions, associations, and more.