Modifiers matching duck:

• common,
• jealous,
• tenacious,
• zany,
• domestic,
• smaller,
• rare,
• large,
• insecure,
• lisping,
• optimistic,
• determined,
• unusual,
• small,
• brash,
• here,
• short-tempered,
• wild,
• paranoid,
• pushy,
• aggressive,
• boastful,
• native,
• indigenous,
• breeding,
• marine,
• slobbery,
• daffy,
• bullying,
• larger,

Simple categories matching aggressive:

• dictator,
• person,
• tetra,
• warrior,
• pest,
• demon,
• tree,
• investor,
• thug,
• condition,
• monster,
• businessman,
• predator,
• environment,
• hero,
• game,
• billionaire,
• gas,
• socialization,
• form,
• athlete,
• bully,
• cargo,
• ceo,
• crime,
• sportsman,
• politician,
• event,
• villain,
• tyrant,
• nature,
• bass,
• lesion,
• behavior,
• policeman,
• interference,
• emperor,
• dog,
• activity,
• comedian,
• dissident,
• father,
• cowboy,
• liquid,
• hunter,
• adventurer,
• equine,
• change,
• sport,
• beast,
• illness,
• lady,
• duck,
• tycoon,
• spy,
• fluid,
• chemical,
• athletics,
• action,
• interaction,
• act,
• track,
• acid,
• boxer,
• vegetable,
• activist,
• compound,
• detective,
• emotion,
• prey,
• shrub,
• bird,
• ant,
• enzyme,
• neoplasm,
• component,
• malignancy,
• solution,
• solvent,
• humorist,
• treatment,
• care,
• soldier,
• caveman,
• outdoorsman,
• pet,
• conflict,
• reagent,
• lymphoma,
• encounter,
• creature,
• terrorist,
• king,
• native,
• feeder,
• cop,
• pioneer,
• plant,
• conqueror,
• campaigner,
• tumor,
• chef,
• therapy,
• intervention,
• offence,
• scientist,
• measure,
• singer,
• outlet,
• fish,
• superhero,
• president,
• crooner,
• explorer,
• placental,
• mayor,
• cichlid,
• disease,
• option,
• ion,
• cancer,
• medium,
• hacker,
• motive,
• behaviour,
• animal,
• grass,
• invertebrate,
• offense,
• agent,
• cleaners,
• child,
• substance,
• womanizer,
• insect,
• actor,
• material,