"Tail dog" has the following two meanings in Cantonese:

Always follow others' steps when doing things. People often have a derogatory connotation.

Dogs that are kept as pets get their name because they like to follow people around.

Later, this word was also used to describe a person who does whatever others tell him to do, a person who has no independent opinions or original ideas, or a person who follows the big boss and acts like a bully.