Well, 'Mama' is a horror movie based on a supposedly true story. It's about two little girls who are left alone in the forest for a long time and are then 'adopted' by a malevolent spirit known as Mama. The story shows how this spirit protects the girls in a very creepy way from the outside world, and the terrifying events that unfold when other people try to interfere with their strange 'family' situation.
It's hard to say for sure. 'Mama' could be inspired by real-life incidents but have a lot of fictional elements added for entertainment value. Or it could be completely made up from the imagination of the creators.
I'm not sure specifically which'mama horror story' you mean. There could be many horror stories related to'mama'. It might be about a haunted mother figure, or some kind of supernatural event involving a mother. Maybe it's a story where a mother is possessed by an evil spirit and starts to act strangely.
Mama (2013) is purely a work of fiction. It wasn't inspired by any real events or people. The plot and characters were imagined by the filmmakers to thrill and scare the audience.