Fictional monsters are those that have been devised by a known author (sometimes a corporation rather than an individual) in a work of fiction or a game in any medium (poem, novels, movies, role-playing games - including MMORPGs, console games, and so on), rather than one that stems from myth or legend (or reality!).
However, sometimes the lines are blurred, as when an author takes a mythical or legendary monster and imbues it with significantly different characteristics. Some fictional monsters take this to the extreme, the gorgon in Dungeons & Dragons being a notorious example.
Many other works of fiction use mythical and legendary monsters in recognisably traditional guises, of course.