Listen, Grýla is such a thing in Iceland, she's literally mentioned on the official Iceland website. She's an ogress who, according to the lore, lives in the Icelandic mountains. The small block of text goes on to paint her as a "dreadful character" who is "part troll and part animal." While this site in particular doesn't give any further physical description, an article from Smithsonian magazine has some choice excerpts from historic sagas and poems.
One says: "Here comes Grýla, down in the field / with fifteen tails on her."
Another: "Down comes Grýla from the outer fields / With forty tails / A bag on her back, a knife in her hand / Coming to carve out the stomachs of the children / Who cry for meat during Lent."
I suppose this is a great moment to tell you what Grýla does. According to Iceland's website and Smithsonian, she hunts down naughty children, throws them into her sack, drags them up the mountain, and then boils and eats them. She apparently even killed her first two husbands! Because she was bored!