Synonyms for Monster:

adj.

large (adjective)
massive.

n.

mare, the impossible, task, salamander, phoenix, headache, a tall order, necessity, the elephant in the room, burden, struggle, chore. animal, sadist, savage, despot, Genghis Khan, psycho, abnormality. juggernaut, macrocosm, biggie, usual, bulk, stonker. great, big, leviathan, high, jumbo, wide, large, whopper, mammoth. boogeyman, tiger, creature, bigfoot, bunyip, kind, bogey, centaur, the Abominable Snowman, bogeyman. elephantine, colossal, immense, kiddie, mountainous, huge, tremendous, gigantic, Brobdingnagian, mighty, vast, walloping, massive, monstrous, fidget, herculean, brood, Pythonic, Gigantesque, brat, Massy, kid, mite, titanic, whopping, crybaby, monumental, cyclopean, Bunyanesque, stupendous, monkey, gargantuan, heroic, kiddy, enormous, mastodonic, prodigious. barbarian (noun)
vandal, brute.
bogey (noun)
boogeyman.
evildoer (noun)
wrongdoer, viper, tyrant, thug, desperado, brute, crook, criminal, hoodlum, bully, vulture, scoundrel, law breaker, thief, vandal, ruffian, evildoer, snake, hooligan, gunman, cutthroat, terrorist, bad guy, tough.
evil spirit (noun)
dwarf, beast, fiend, dragon, satyr, hellcat, demon, Nixie, sprite, nymph, siren, sylph, freak, archfiend, hell hound, hell-raiser, goblin, gnome, imp, lamia, Genie, hellion.
ghost (noun)
chimera, bugbear, shadow, ghoul, frankenstein, incubus, apparition, daemon, hobgoblin, terror, bugaboo, illusion, spirit, dracula, werewolf, Phantasm, ghost, specter, ogre, vampire, bogey man, succubus, horror, nightmare, phantom, myth.
giant animal; supernatural being (noun)
brute, abnormality, centaur, behemoth, horror, fiend, beast, ogre, Lusus Naturae, hellion, leviathan, mammoth, colossus, phoenix, savage, werewolf, dragon, demon, freak, giant, monstrosity, frankenstein.
monster (noun)
colossus, demon, fiend, Lusus Naturae, freak, monstrosity, giant, behemoth, goliath, teras, ogre.

Other synonyms:

Genghis Khan, Gigantesque, leviathan, stonker, Massy, macrocosm, despot, psycho, sadist, large, Bunyanesque, walloping, Pythonic, Brobdingnagian, cyclopean, mastodonic. juggernaut, gargantuan, vast, huge, mountainous, gigantic, mammoth, tremendous, tiger, whopper, prodigious, creature, jumbo, colossal, enormous, immense, titanic, whopping, monumental, bulk, elephantine. heroic, stupendous, monstrous, herculean. mighty, massive. savage, animal. Other relevant words:
centaur, elephantine, sadist, cyclopean, gigantic, juggernaut, abnormality, great, animal, mastodonic, Bunyanesque, salamander, monkey, phoenix, teras, creature, bulk, goliath, Brobdingnagian, enormous, leviathan, behemoth, mighty, herculean, colossus, prodigious, monumental, stupendous, titanic, whopping, psycho, tremendous, monstrous, boogeyman, heroic, savage, large, jumbo, big, immense, tiger, stonker, massive, mammoth, Pythonic, huge, whopper, despot, Gigantesque, colossal, vast, walloping, bogey, Massy, monstrosity, bogeyman, giant, gargantuan, mountainous, Lusus Naturae.

Usage examples for monster

  1. As for that monster fish, there are times, even when the fishermen are not telling me about him, that I believe he exists. – Old Plymouth Trails by Winthrop Packard
  2. Two hussars riding before Banfy fell at one blow from the monster one to the right, the other to the left of his horse. – The Golden Age in Transylvania by Mór Jókai
  3. Whence, then, comes this monster of a melon? – Eastern Tales by Many Story Tellers by Various