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The Nightmares were originally harmless and happy dreams constructed for children by the Sandman. However, Pitch corrupts and manipulates the magical golden sand they are made of, turning it into black sand and rendering them Nightmares. They now take the form of sinister black horses that have the power to sense or "smell" nearby fear.
During the film's climax, most of, but not all, the Nightmares were turned back into dreams by the courage of children who believed in the Guardians. The ones that remained were last seen when they dragged Pitch into his lair after sensing his fear following his defeat.
Despite their transformation, the Nightmares seem to retain some of the same comedic and lighthearted habits they had as dreams.
Nightmares in the official trailer.
Nightmares form in the night sky.
Nightmares approach Jack Frost and the Sandman.
Pitch and the Nightmares corner the Guardians.
The Nightmares being attracted by Pitch's fear.
The head Nightmare is named Onyx.
There is a play on words with the Nightmares, as they take the form of female horses, which are known as mares.
Pitch's statement that the Nightmares "smell fear" and their turning on him at the end of the movie because he felt fear, indicates that they are truly loyal only to fear itself and they will seek out and terrorize anyone who feels it.