Does a veterinary doctor treat only domestic animals?

No, veterinarians provide medical care for all kinds of animals. Pets, both domesticated species and captive bred or caught wild species, are all seen by veterinarians - this includes cats and dogs as well as snakes, birds, reptiles, fish, etc.

Veterinarians also care for livestock species, including horses, cattle, sheep, goats, pigs, llamas, alpacas, camels, water buffalo, yak, etc.

Finally, there is a subspecialty of veterinary medicine that focuses on wild animals, including those housed at zoos and aquariums as well as treating injured or diseased animals in the wild (mostly endangered animals and only on an as-needed and as-safe basis).