Nightmares, also known as Onyx, are horse-like beings created out of blacksand -- originally Sandman's dreamsand poisoned and manipulated by Pitch. Onyx is Pitch's pet, formerly Cupcake's own dream of an unicorn.
In the Rise of the Guardians App, Dash n Drop, there's also another nightmare creature that resembles a large bat.
In the Rise of the Guardians: The Video Game, there are many more Nightmare creatures based on scary animals. Wolves, Bats, Rats, Killer Lobsters, Giant Squids, Wasps, Hideous Monkeys, Nightmare Orchids and more.
- Onyx's fate was that Cupcake restored him back to a Dreamsand Unicorn.
- Onyx's name was never mentioned in the film, except in the film's screenplay.
- They are the sixth characters to start off as an antagonist but cause the death or defeat of the main antagonist after Mr. Tweedy, Dragon, King Harold, Nana and The Terror.