It is a shapechanger and has no set form. When feeding, It takes the form of whatever Its victim fears most. Throughout the novel, It appears most often in the guise of Pennywise the Dancing Clown. Pennywise appears human at first glance, but on closer inspection anomalies (such as abnormal or changing eye color) are apparent.
Its true form on the physical plane is a large spiderlike creature. This is not Its true form, but is instead the closest approximation the human mind can perceive when looking at It in this state. Its real body is the Deadlights, a mass of writhing orange light lying within an otherdimensional space called the Macroverse. Looking at the Deadlights can cause catatonia or even death.