General terms:
* Beast: Emphasizes the animalistic nature of the creature.
* Creature: A more neutral term, but can also suggest something unnatural or frightening.
* Abomination: Implies something morally repulsive or physically grotesque.
* Fiend: A more formal term for a demon or devil.
* Behemoth: A large and powerful creature.
Specific types of monsters:
* Demon: A supernatural evil being.
* Devil: A malevolent spirit.
* Ghoul: A creature that feeds on corpses.
* Vampire: A creature that feeds on blood.
* Werewolf: A human who transforms into a wolf-like creature.
More metaphorical terms:
* Terror: Focuses on the fear and dread the creature inspires.
* Scourge: Emphasizes the destruction caused by the creature.
* Nightmare: Evokes a sense of fear and dread.
* Freak: Implies something unnatural or deformed.
Ultimately, the best substitute depends on the specific situation. Consider the context, tone, and desired effect when choosing a word.