Nightmare

Nightmare is a three year-old, demonic horse and the near-inseparable pet of Headless Headmistress Bloodgood.

Physical Description
Nightmare is a blue horse with purple manes and the ability to breathe fire. She is prone to bouts of panic and fear at even the suggestion of something bad about to happen.

Volume 2
In "Frightday the 13th", all the bad luck and tales of a monster lurking the halls of Monster High at midnight were enough for Nghtmare to poop burning charcoals. Ghoulia Yelps extinguished them. Since Bloodgood herself was very eager to leave the building before the monster would awaken, Nightmare swiftly responded to her order to saddle up and jumped through the window.

When Abbey Bominable and Frankie Stein were sent to Bloodgood's office in "Abominable Impression", Nightmare was just assisting Bloodgood in doing her makeup. When Bloodgood put bracelets on the two students to force them to stay together for the rest of the day, she added that if Abbey and Frankie would not adhere to the bracelets' distance limits, they'd be cursed with eternal homework. This frightened Nightmare badly.

Nightmare was present when Bloodgood in "Dueling Personality" told the Fear Squad that Nefera de Nile would be their coach until Monster Mashionals. In "Neferamore", she accompanied her master to the gym to help the new coach get acquainted with her task, then set fire on a door in order to leave. Again, Ghoulia extinguished the flames.

Fright On!
When Monster High was preparing for a merge with two other schools, one of the staff members, Ms. Crabgrass, spent a lot of time working with her. This was difficult for Nightmare, who was constantly trying to take a bite out Crabgrass' leaf skirt, but of course not allowed to do so.

Upon the success of the merge, Crabgrass contacted her boss, Van Helscream, who immediately traveled to Monster High. Nightmare immediately understood he was up to no good, but could not prevent him from turning Bloodgood into a statue. In fact, as the only witness, Nightmare couldn't even save herself and was soon captured. However, the panicked way in which she acted towards Crabgrass, who had changed her appearance into that of Bloodgood's to cover up the disappearance, managed to make Frankie Stein realize Bloodgood had been replaced. When Frankie subsequently figured out Bloodgood had been petrified and gained access to Bloodgood's office to search for the cure, Nightmare betrayed the location of the antidote, ensuring Frankie got it before Crabgrass and van Helscream returned.