Maras are also known as mares in English. Mara is a very old word for them, and I decided to go with that one as people wouldn’t start thinking of horses when using that form. 😉

A mara can best be described as a mix between a female werewolf and a succubus, that’ll sneak up on their unknowing sleeping victim and give them nightmares.
In daytime they are normal human females, but come night time they can transform into mares, and ride on top of their sleeping victim. This will give the victim nightmares (yup, at night the mare comes. It’s not a coincidence that the word for nightmare is just that).
The mara will sit on the sleeping victims torso. Should the victim wake up while the mara is riding on them, they’ll find themselves unable to move while the nightmare continues (This is also known as night terrors. It’s an actual thing happening for some people).
Maras favourite victims are young men they find attractive. But horses or even trees could be targeted, if they for some reason felt like it. You knew they had ridden the horse, if at morning it was covered in sweat. And the trees would be distorted and grow into a grotesque shape if the mara attacked it.
Mara’s could be kept away with iron and certain magic signs. Also, they’d enter a home through small openings or holes, so you could trap a mara, if the opening it entered through was covered up while it was in your house. This also would take away it’s magic and it’d transform back into it’s human shape. Typically, the mara would turn out to be a beautiful young maiden, in stories where this happened. So the young man that had been targeted would instantly marry her, and all was well… Until the hole that had been covered up somehow was uncovered again, and the mara instantly would flee away from her husband (jailer), as the binding had been broken.