Horror is a genre literal but her appearance doesn't fit her name. As she appears as a cute little blonde girl in a pink dress. But those looks are still true to her name. For she is a terrifying little girl. She can even scare Kevin Thorn. Whenever she does talk she speaks of horrifying and disturbing things. She was called upon by Kevin; who was under the influence of Writer's Block. However, he still found even with the genres here, that they were boring. Bigby, Snow, Gary, and Mister Revise; along with the Page Sisters, had come to stop him from doing further damage to reality. Kevin sent the Genres, including Horror to guard his house and fight off the others.
Horror was most likely enjoying all the fighting, but she took great interest, as did Literature, in the battle when they heard Mr. Revise was near. She held a grudge against him for editing the Odyssey and hoped to kill him for it. Bigby, who had been put through much humiliation and transformed into a girl to further it, lost his patience. While Robin, Priscilla, and Hillary fought off the others, he snuck around trees and attacked all the Genres. Horror was the first to see her(Bigby who was still stuck in his little girl form) and immediately ordered the others to kill her(Bigby). Quoting the sweeter they are, the more danger they are; since she spoke from personal experience, given her appearance.
However, Blockbuster and Western refused, as it was a rule not to kill a little girl, especially a child. That would be a mistake as Bigby would show no mercy and kill Horror and the rest of the Genres. For the moment, after defeating Kevin, Horror and the rest of the Genres had apparently recovered. Or it meant they were resurrected by Kevin or simply replaced. They were all sent into a new world where Kevin could do what he want and leave the mundy world alone.