Mare (folklore) or nightmare, a demon of Germanic folklore
Night Mare, a 1983 novel by Piers Anthony
Nightmares (novel series), a young-adult horror fiction series by various authors published by HarperCollins (UK) under their "Lions" imprint between 1992 and 1996
The Nightmare, a painting by Henry Fuseli
Nightmare (Marvel Comics), a fictional supernatural being
Nightmare, a painting by Nicolai Abildgaard
Nightmare, a horror comic from Skywald Publications
Nightmare, a 1970 novel by Russell H. Greenan
Nightmares!, a young adult book series co-authored by Jason Segel and Kirsten Miller
Computing and gaming
Nightmare (Atmosfear series), a board game
Nightmare (Devil May Cry), a demon in the video game series Devil May Cry
Nightmare (Dungeons & Dragons), a creature in the role-playing game Dungeons & Dragons
Nightmare (Kirby), a villain in the video game Kirby's Adventure
NightMare (scareware), an early scareware program
Nightmare (Soulcalibur), a villain in the fighting game series Soul
Nightmare LPMud, a 1992 defunct text-based online role-playing game
Nightmare, a boss in the video game The Legend of Zelda: Link's Awakening
Nightmare, a boss in the video games Metroid Fusion and Metroid: Other M
Nightmares, monsters in the role-playing game Dream Craft