In The Spiderwick Chronicles, the brownie living in the estate walls that becomes a boggart when angered is Thimbletack.
Thimbletack is a small house brownie who has lived for years inside the walls of Spiderwick Estate. He is initially associated with helpful household work, but the Grace children’s interference with his nest and later conflicts cause him to become angry and retaliatory. In the series’ mythology, that anger triggers his transformation into a boggart, a more dangerous and troublesome form.
The character’s shifting nature is central to the story’s treatment of traditional folklore. Brownies are often portrayed as helpful domestic spirits, while boggarts are commonly associated with frightening mischief or harmful household disturbances. Thimbletack combines both ideas in one character rather than representing two separate creatures.
Readers sometimes confuse him with Mulgarath, the main ogre antagonist, or Hogsqueal, a hobgoblin-like creature encountered later. Redcaps are also dangerous faerie beings in the books, but none of those characters is the brownie attached to Spiderwick Estate.