What was the name of the demon that possesses the young girl, Regan MacNeil, in the movie "The Exorcist"?

In the 1971 novel "The Exorcist" and the film based on the novel, "Pazuzu" is the evil spirit that possesses the young girl Regan MacNeil. He, also, reappears and is the main protagonist in the 1977 sequel "Exorcist II: The Heretic" and "The Exorcist" horror novels. In "Exorcist II: The Heretic", "Pazuzu" is both named as the demon antagonist of Regan and the unwitting helper of Father Philip Lamont (Richard Burton), as he seeks to free Regan from the demon's grip.

"Pazuzu" was also featured in the following:

The 2014–15 NBC television series "Constantine" in a two-part episode "The Saint of Last Resorts".

"Pazuzu" (voiced by Kevin Michael Richardson) made an appearance in a 2017 episode of "The Simpsons', 'Treehouse of Horror XXVIII", possessing Homer, (briefly) Maggie (until the end), and Bart (at the end) in a spoof of "The Exorcist".

In the ancient Mesopotamian religion, "Pazuzu" (also called Fazuzu or Pazuza) was the king of the demons of the wind. He also represented the southwestern wind and was the bearer of storms and drought. He was the brother of Humbaba, the guardian of the Cedar Forest, where the gods lived, and son of the god Hanbi (in Sumerian and Akkadian mythology Hanbi was the god of evil and the god of all evil forces).