This user chose to "Post as a guest" rather than registering an account. The name shown is the name they gave at the time they posted.
Such users haven't set a password, verified an email address, etc., and are only authenticated via a browser cookie. If the cookie is lost, the user can no longer edit or accept answers to their posts and has no way to log in or recover access to their contributed content.
See also: Why should I register my account?