She is prone to bouts of panic and fear at even the suggestion of something bad about to happen. She usually breathes fire.
Nightmare is a bright blue horse with a blue-and-purple-pink mane, red-and-white-pink eyes that are red sclera (or "whites of the eyes") but they are not white, and are red, white-pink or pink-white irises, and blood-red, black-red pupils instead of fully black pupils; blue hooves, a blue tail with pinkish purple colors on it, a dark navy blue bridle over her head and muzzle with Skullettes on it, and long black eyelashes. She is large in size despite being only three years old.
She is the only pet to a romantic interest because like all the student at school she fell on the charm of Archer.
Nightmare's pet is Headless Headmistress Bloodgood.
- July 30, 2010: Nightmare makes her audial webisode debut in "Totally Busted".
- May 12, 2011: Nightmare makes her visual webisode debut in "Frightday the 13th".
- Mid June, 2011: Nightmare makes her diary debut in Clawdeen's 'School's Out' diary.
- April 13, 2012: Nightmare makes her 3D-animated debut in "Escape From Skull Shores".
- June 2013: Nightmare's doll is released with Headless Headmistress in a two-pack
- Nightmare's name, obviously, is a play on the fact that a nightmare is something unpleasant and that a mare is an adult, female horse. But "adult, female horse" is not actually what "mare" in the word "nightmare" refers to. In Germanic folklore, a mare or nightmare is a demonic spirit thought to sit on people when they are asleep and cause them a bad rest, which can manifest itself as a bad dream or even sleep paralysis. It is a coincidence that this creature is called a mare and a female horse is called a mare too, as the two words have different etymological histories.
- In "Fright On!", it is released that she is 3 years old.