Here are ten creatures that show up in Norwegian folklore. Some of these are very widely known, others... not so much.
The tusse is an underground dweller who generally only comes aboveground during twilight or at night. They can turn invisible and are usually dressed in grey... otherwise they vary a lot in appearance and behaviour.
The dragdoll (not to be confused with a ragdoll) is a helping spirit that takes on the form of a doll. Anyone who owns a dragdoll will never lack for money -- because the doll goes out and finds treasure or just steals from the neighbours.
The marmenill is much like a mermaid, except she looks like a young child. The marmenill gets cold easily and likes to receive blankets and clothes to keep her warm when above the water. She's very clever and can answer any question.
The nisse is one of the most well-known and beloved Norwegian creatures. He's a kind of gnome who usually lives around people -- most famously on farms -- and brings wealth and prosperity to the humans if treated well.
The fossegrim lives in or around waterfall, and is the best fiddle-player imaginable. It's said that anyone who wants to become a really GOOD musician has to talk a fossegrim into teaching him... which the fiossegrim will do if he's paid well.
The nix is a mysterious and often sinister figure who lives in lakes and who likes to lure people to him. He's a shapeshifter and can take on any form, but often he just looks like a clump of grass and twigs floating on the surface.
Trolls are plentiful in Norwegian folklore. They live in mountains or forests, and come in so many variants that it's impossible to keep count. Most of them are stupid and evil brutes -- but there are exceptions.
The huldra is a beautiful woman who lives in the forest. She looks completely human, except for the fact that she has the tail of a cow. The huldra is a seducer who lures people with her hypnotic song... but she can be kind and helpful too.
The mara is a spirit who roams the night and seeks out sleeping humans to give them nightmares. Some stories portray the mara as a human woman under a curse -- a man who's cursed becomes a werewolf, and a woman becomes a mara.
Trollcats aren't real cats; they're more like artificial creatures made by, and in service to, a witch. The trollcat can look like a big, shaggy cat, with one yellow and one blue eye -- or like a grey ball of yarn that rolls around all by itself.