* Do you mean a specific breed of dog? There are many large dog breeds, such as Great Danes, Mastiffs, Saint Bernards, etc.
* Are you referring to a specific company or product? There is a company called Big Dog that makes clothing and accessories.
* Are you asking about a general term? "Big Dog" can be used as slang for someone who is dominant or in charge.
Once I know what you're looking for, I can help you find the information you need.