Legends are things that are so widely-spread and well-known to have been around for so long, that it would be silly to question the truth of their existence for even two seconds. Therefore, all legends are true.
Many different cultures have many different legends. Luckily, with the internet, we can now believe all the legends of all the cultures simultaneously, even if they seem contradictory or mutually exclusive at first.
They are usually ancient and very powerful. If you are lucky enough to see a legend, you will forevermore have stories and good conversation topics at parties making you popular and sexy.