Like minor NPCs, monsters and other threats (like a storm, a spreading fire, or a squad of armored minions) are written up as characters, but are usually simpler than a PC. You only need to define what is absolutely necessary. Unlike minor NPCs, these threats can be defined really in any way. Break the rules. Give them whatever combination of aspects, skills, stunts, stress, and consequences it will take to make them dangerous, and think about what sort of difficulties they will present to the PCs when determining their ratings.