A story that specific cultures use to explain their origins and the origins of the world. Think along the lines of creation myth.

The origins of these myths are not known. They are probably very ancient.

The vampire myth with the greatest claim to antiquity would be that of Lilith in the Garden of Eden. This myth quotes a ancient setting but may be of a more recent origins.

I guess it is because it deals with evil and how all evil comes from there.

No. Orisha are a concept of the Yoruba religion, which has African origins. Mermaids are primarily a Western myth.