(informal, childish) A terrifying and dangerous creature.

(informal, childish) A bizarre or whimsical creature.

A cruel, heartless, or antisocial person, especially a criminal.

(figurative) A badly behaved child, a brat.

(informal) Something unusually large.

(informal) A prodigy; someone very talented in a specific domain.

(gaming) A non-player character that player(s) fight against in role-playing games.

(transitive) To make into a monster; to categorise as a monster; to demonise.

(intransitive) To behave as a monster to; to terrorise.

(chiefly Australia) To harass.

(UK, live action roleplaying games) To play (a series of) non-player characters as directed, without having the responsibility of organising the game itself; generally not limited to playing literal monsters or hostile combatants.

(informal) Very large; worthy of a monster.

(informal) Great; very good; excellent.