In English folklore, the shape-shifting household spirit that can become a terrifying creature is a boggart.
Boggart is a broad English folkloric term for a malevolent or troublesome supernatural being. In northern England, especially Lancashire and Yorkshire, a household boggart was believed to inhabit a particular home or farm. It might make noises, steal objects, disturb sleepers, spoil food, or cause illness. Some traditions said it could follow a family if they moved house.
The creature’s form was not fixed. A boggart could be imagined as a goblin-like figure, an animal, an apparition, or another frightening shape. This flexibility reflects the word’s wider use for local spirits attached to places such as hills, roads, and marshes. It is therefore not simply another name for a brownie: brownies were generally helpful household beings, while boggarts were dangerous or spiteful.
Modern readers may know boggarts chiefly from J. K. Rowling’s Harry Potter books, where they take the form of a person’s worst fear. That specific fear-shape idea is a modern fictional development, although it draws on the older tradition of a changeable, unsettling spirit.