Simple categories matching mysterious:

• phenomenon,
• thing,
• tune,
• creature,
• being,
• artefact,
• entity,
• crystal,
• relic,

Members with property mysterious:

• loch-ness_monster,
• culture_hero,
• grey_cap,
• incommensurable_value,
• dragon,
• moon,
• brain,
• sun,
• god,
• unconscious,
• lyric,
• sphinx,
• sash,
• pigment,
• neocon,
• electron,
• database,
• lunar_goddess,
• seasoning,
• dark,
• self,
• sky,
• lymphocyte,
• ufo,
• giant_octopus,
• nucleus,
• proposition,
• flavoring,
• sea,
• ocean,
• eye,
• dolmen,
• song,
• leucocyte,
• bible,
• harp,
• time_traveler,
• esp_user,
• magnetic_mass,
• menhir,
• octopus,
• fresh_blood,
• mail,
• sea_monster,
• dark_crystal,
• giant_bird,
• ounce,
• bacillus,
• seahorse,
• big_foot,
• germ,
• key,
• title_song,
• platonic_form,
• alien,