King Ghidorah is the creature codenamed Monster Zero in the 2019 film Godzilla: King of the Monsters.
The film uses “Monster Zero” as a designation for King Ghidorah, a three-headed extraterrestrial dragon and one of Godzilla’s most famous enemies. The name connects the creature to the wider Japanese kaiju tradition established by Toho, where Ghidorah first appeared in 1964.
In the film’s story, the eco-terrorist group known as the Alan Jonah organization awakens several Titans. King Ghidorah’s arrival destabilizes the world’s natural order and makes him the principal threat faced by Godzilla. Rodan and Mothra also play major roles in the conflict.
The title Pandora’s box is sometimes used metaphorically for the film’s release of ancient Titans, but the question’s specific answer is the creature’s identity. Monster Zero is not a separate monster from Ghidorah; it is the in-story codename used for him.