Staff may assign themselves or others custom titles, or you may assign your own custom title if you win MotM (Member of the Month) or, now, MotS (Member of the Season).
Only staff members or MOTS are able to change there custom titles. Regular users arnt really able to unfortunally. Usually the people you see with custom titles are either staff, or have been in MOTSHere ye' are. I believe ye'll find these two answers satisfyin', aye?
Thank ye, I may ask a staffer to change mine.