The Nightmares are Pitch's minions from the 2012 computer-animated DreamWorks film, Rise of the Guardians. The nightmares were originally harmless and happy dreams constructed for children by the Sandman, but have been corrupted by Pitch, who found a way to turn dream sand into nightmare sand. They now take the form of sinister 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 light-hearted habits they had as dreams.