Pennywise

It (referred to as IT) is an ancient alien/eldritch monster and the title character and main antagonist of the 1986 novel the by same name (and subsequent television and film adaptations). It is a shapeshifting creature known as a Glamour and is billions of years old. Although it lived on planet Earth for many years, it originated in a void/dimension outside the regions of space known as the Macroverse. Since its true identity is unclear, due to its shape-shifting abilities and being from another universe, its real name and species are called Deadlights, but they don't know this, thus why it is named IT. It can morph into any other person, animal, or object (including combinations thereof). This ability is useful for A) appearing as the target's loved ones or friends to lure them into a trap or B) appearing as the target's fear. However, It’s favorite and most common form is that of a circus performer named "Pennywise the Dancing Clown". Its real name (if indeed, it has one) is unknown. Although, several times in the novel, It calls itself by the name Robert “Bob” Gray. This may be an allusion to real life child cannibal Albert Fish, who reportedly used the name "Robert Gray" as an alias.

Origins
It originated in an undiscovered void containing and surrounding our entire Universe, another universe referred to as the "Macroverse." (A concept related to Todash Darkness from The Dark Tower Novels). Other evidence from this series suggests It is the creation of a separate omnipotent creator referred to as "the Other" (who may be the entity Gan). This "Other" helps the defeat of it at the end, and may also be responsible for the creation of "The Turtle", who is IT's natural enemy. In It the novel, the Turtle refers to It as his "brother". It may be one of, or a Twinner of, the six greater demon elementals mentioned by Mia in Song of Susannah (as the Spider is not one of the Beam Guardians). It's possible that there are other creatures of the same species as It, due to the fact that It is revealed to be pregnant.

History
It arrived on Earth in a massive cataclysmic event similar to an asteroid impact, landing in a location in North America that would eventually become Derry, Maine. Once people settled over this location, It adopted its usual pattern of a 27-30 year hibernation, waking to kill and eat. Each awakening and return to hibernation is sometimes marked by a violent act, such as a mass murder or weather event. In the period between, a series of child murders occurring in Derry are never solved. It is outwardly explained that a series of murders, no matter how gruesome, don't get reported if they happen in the small town. However, the real reason is that the influence of It prevents anyone from investigating too deeply.

(2017 film)
The 2017 film places those events in 1989.[note 2] This adaptation also introduces new events that cannot be placed canonically: Derry starts as a beaver camp until 91 people vanish. The rumored cause is plague or slaughter by Indians, but there are no signs of an attack. The only evidence is the presence of bloody clothes at the Well House.