The expression "dog rider" is not a universally accepted or used term, and there is no clear definition or context as to why motorcycles are called "dog riders."

This statement may be a slang or joke used among certain regions, groups or individuals. or a specific expression, but it is not a widely recognized general term.

Usually, motorcycle riders are called "motorcyclists", "riders", "motorcycle enthusiasts", etc. These terms more accurately describe their relationship to motorcycles and who they are.