Real Dictionary

dog
countable noun: A dog is a very common four-legged animal that is often kept by people as a pet or to guard or hunt. There are many different breeds of dog.
countable noun: You use dog to refer to a male dog, or to the male of some related species such as wolves or foxes.

dog in American English
any of a large and varied group of domesticated canines (Canis familiaris) often kept as a house pet or used for hunting, guarding people or property, etc.
noun: a domesticated canid, Canis familiaris, bred in many varieties