Maras: An all female race of werewolves, created through a rather weird process that I'll gloss over for simplicity, will probably use as a night creature/semi-megabeast.
I've read that the Mara is a kind of spirit causing paralyzation, and that the same process that makes a female baby a Mara makes a male baby into a werewolf. Never heard about any all-female werewolves.
I found some stuff referring to Mara as being spirits that caused nightmares, which is I think meant to be the origin of the word nightmare. Quite a common myth from Europe, as I recall the germans called it Alpdrucken and the English term is Hagridden. The one I'm using described Maras as being the daughters of women who had used a ritual of crawling through part of the membrane from a horse foals birth to avoid the pain of childbirth. All the sons of these women were shamans and the daughters were werebeasts with the heads of wolves. I'm using the latter because it's more Fun, though I may put nightmares in as well once I settle on a way to depict them.
Nokken: Water spirits that drown men and eat them, supposed to be able to shift between man and horse shape. I'll probably just use them as aquatic horses that hate people.
Sounds like a mash-up between Nokken and Bäckahästen.
Having now looked up the Bäckahästen it would seem that the source I was reading lumped them together under the category of Neck, or nyx and nixies. The Bäckahästen/bækhesten tricking people into riding it and then drowning them, and the näck/näkki/nøkk/nøkken/strömkarl/Grim/Fosse-Grim using music to lure people, women and children especially into water to drown them. The article did make me remember the Kelpie from my own country's old folktales. Basically the same as the Bäckahästen, and there are quite a few similar myths from the rest of Britain.
I could split them into different creatures, or just flip a coin and use one since they'd serve the same basic function.