The Outsider appears in The Outsider novel and TV adaptation. The monster is a shape-shifting creature that feeds on people's pain, sorrows and fears.
Stephen King fans may already be familiar with the dark void between worlds known as "Todash space" or the Macroverse. Some are still debating whether these places are actually the same, but basically, nearly all of Stephen's nightmare-inducing monsters come from this place — including Pennywise, who feeds on fear, and Dandelo, a creature of the same species who feeds on laughter.
Since the Outsider has a similar ability to feed on emotions, it's very likely that the creature is of the same species and therefore, comes from that same void.
As for its true form, it's revealed in the story that the creature is made up of worms that can shift into different people and carry their DNA. That should make for a nightmarish visual in the series...