Lewis & Short

monstrum, i, n. [moneo].

  1. I. Orig. belonging to relig. lang., a divine omen indicating misfortune, an evil omen, portent (syn.: ostentum, prodigium, portentum): quia ostendunt, portendunt, monstrant, praedicunt, ostenta, portenta, monstra, prodigia dicuntur, Cic. Div. 1, 42, 93; cf.: monstrum dictum velut monestrum, quod moneat aliquid futurum; prodigium velut praedicium, quod praedicat; portentum quod portendat; ostentum, quod ostendat, Paul. ex Fest. p. 140 and 138 Müll.; cf. id. ib. p. 157: hic est nullum (dubium) quin monstrum siet, Enn. ap. Non. 469, 6 (Trag. v. 326 Vahl.): monstra deūm, Verg. A. 3, 59; cf. Ov. M. 15, 571.
  2. II. Transf., a monster, monstrosity (whether a living being or an inanimate thing).
    1. A. Ofliving beings: monstrum hominis, you monster of a man, Ter. Eun. 4, 4, 29: monstrum advenit, id. ib. 5, 2, 21: horrendum, of Polyphemus, Verg. A. 3, 658.
      Also with respect to character: en monstrum mulieris, that monster of a woman, Plaut. Poen. 1, 2, 64: nulla jam pernicies a monstro illo atque prodigio comparabitur, Cic. Cat. 2, 1, 1: fatale, of Cleopatra, Hor. C. 1, 37, 21: tune etiam, immanissimum ac foedissimum monstrum ausus es? Cic. Pis. 14, 31: hominum, monsters of men, Gell. 17, 1, 1.
      Of beasts: succinctam latrantibus inguina monstris, i. e. canibus, Verg. E. 6, 75: quae plurima terrae monstra ferunt, id. G. 1, 185.
    2. B. Of inanim. things, of the sea, Verg. A. 5, 849: infelix, of the Trojan horse, id. ib. 2, 245.
      Of the ship Argo, Cat. 64, 15: non mihi jam furtum, sed monstrum ac prodigium videbatur, Cic. Verr. 2, 3, 73, § 171: monstra narrare, dicere, to relate wonders, prodigies, marvels: mera monstra narrabat, Cic. Att. 4, 7, 1: dicere, id. Tusc. 4, 24, 54: ac portenta loqui, Lucr. 590: totaque jam sparsis exarserat insula monstris, Val. Fl. 2, 248: in vitā suā fecit monstra, he did wonderful things, Vulg. Eccl. 48, 15; Sap. 19, 8.