Online Etymology Dictionary

monster: c.1300, "malformed animal, creature afflicted with a birth defect," from O.Fr. monstre, from L. monstrum "monster, monstrosity, omen, portent, sign," from root of monere "warn" (see monitor). Abnormal or prodigious animals were regarded as signs or omens of impending evil. Extended c.1385 to imaginary animals composed of parts of creatures (centaur, griffin, etc.). Meaning "animal of vast size" is from 1530; sense of "person of inhuman cruelty or wickedness" is from 1556. In O.E., the monster Grendel was an aglæca, a word related to aglæc "calamity, terror, distress, oppression."
monstrosity: 1550s, "abnormality of growth," from L.L. monstrositas "strangeness," from L. monstrosus, a collateral form of monstruosus (cf. Fr. monstruosité); see monster. Sense of "quality of being monstrous" is first recorded 1650s. Noun meaning "a monster" is attested from 1640s.
sphinx: c.1421, "monster of Gk. mythology," from L. Sphinx, from Gk. Sphinx, lit." the strangler," a back-formation from sphingein "to squeeze, bind" (see sphincter). Monster, having a lion's (winged) body and a woman's head, that waylaid travelers around Thebes and devoured those who could not answer its questions; Oedipus solved the riddle and the Sphinx killed herself. The proper plural would be sphinges. Transf. sense of "person or thing of mysterious nature" is from 1610. In the Egyptian sense (usually male and wingless) it is attested from 1579; specific reference to the colossal stone one near the pyramids as Giza is attested from 1613.
kraken: "monster of the North Sea," 1755, from Norw. dial. krake.
moon-calf: also mooncalf, "abortive, shapeless, fleshy mass," 1560s, attributed to the influence of the moon. Later 16c., "deformed creature, monster."
gila monster: Heloderma suspectum, 1877, Amer.Eng., from Gila River, which runs through its habitat in Arizona.
Frankenstein: allusive use dates to 1838, from Baron Frankenstein, character in Mary Shelley's 1818 novel "Frankenstein: The Modern Prometheus." Commonly used, mistakenly, to mean the monster he created, and thus franken- extended 1990s as a prefix to mean "non-natural."
teratology: "study of marvels and monsters," 1678, from comb. form of Gk. teras (gen. teratos) "marvel, monster" + -logy.
monstrous: mid-15c., "unnatural, deviating from the natural order, hideous," from L. monstruosus "strange, unnatural," from monstrum (see monster). Meaning "enormous" is from c.1500; that of "outrageously wrong" is from 1570s.
blatant: 1596, in blatant beast, coined by Edmund Spenser in "The Faerie Queen" to describe a thousand-tongued monster representing slander; probably suggested by L. blatire "to babble." It entered general use 1650s, as "noisy in an offensive and vulgar way;" the sense of "obvious, glaringly conspicuous" is from 1889.
Rahab: name of a Biblical monster, from Heb. rahab, lit. "storming, against, impetuous," from rahabh "he stormed against" (cf. Arabic rahiba "he feared, was alarmed").
Fomorian: "monstrous race in Irish mythology," 1876, from Ir. fomor "pirate, monster," from fo "under" + mor "sea." Cognate of Gaelic famhair.
manticore: c.1300, from L. manticora, from Gk. mantikhoras, corruption of martikhoras, said to be from an O.Pers. word for "man eater," cf. martiya- "man" + root of khvar- "to eat." Fabulous monster with the body of a lion, head of a man, porcupine quills, and tail or sting of a scorpion.
Scratch: in Old Scratch "the Devil," 1740, is from earlier Scrat, from O.N. skratte "goblin, monster," a word which was used in late O.E. for "hermaphrodite" (cf. O.H.G. scrato "satyr, wood demon").
satyr: woodland deity, companion of Bacchus, late 14c., from L. satyrus, from Gk. satyros, of unknown origin. In pre-Roman Gk. art, a man-like being with the tail and ears of a horse; the modern conception of a being part man, part goat, is from Roman sculptors, who seem to have assimilated them to the fauns of native mythology. In some Eng. bibles used curiously to translate Heb. se'irim, a type of hairy monster superstitiously believed to inhabit deserts.
orc: "ogre, devouring monster," O.E. orcþyrs, orcneas (pl.), perhaps from a Romanic source akin to ogre, and ult. from L. Orcus "Hell," a word of unknown origin. Revived by J.R.R. Tolkien (1892-1973) as the name of a brutal race in Middle Earth.
"But Orcs and Trolls spoke as they would, without love of words or things; and their language was actually more degraded and filthy than I have shown it." ["Return of the King," 1955]
Minotaur: c.1385, from Gk. minotauros, from Minos, king of Crete + tauros "bull." A flesh-eating monster half man, half bull, son of Pasiphæ, wife of Minos, and a bull.
afanc: cattle-devouring aquatic monster in Celtic countries, from Celt. *abankos "water-creature," from *ab- "water" (cf. Welsh afon, Breton aven "river," L. amnis "stream, river," which is of Italo-Celtic origin).
ogre: "man-eating giant," 1713, hogre (in a translation of a Fr. version of the Arabian Nights), from Fr. ogre, first used in Perrault's "Contes," 1697. and perhaps formed by him from It. orco "demon, monster," from L. Orcus "Hades," perhaps via an It. dialect. In Eng., more literary than colloquial. The conjecture that it is a from Byzantine Ogur "Hungarian" or some other version of that people's name (perhaps via confusion with the bloodthirsty Huns), lacks historical evidence.
chimera: late 14c., from L. Chimaera, from Gk. chimaira, a fabulous monster (with a lion's head, a goat's body, and a serpent's tail), supposedly personification of snow or winter, orig. "year-old she-goat," from cheima "winter season." Meaning "wild fantasy" first recorded 1587.
"Beestis clepid chymeres, that han a part of ech beest, and suche ben not, no but oonly in opynyoun." [Wyclif, "Prologue"]
Related: Chimeric (1650s); chimerical (1630s).
loch: late 14c., from Gael. loch "lake, narrow arm of the sea," cognate with O.Ir. loch "body of water, lake," Bret. lagen, Anglo-Ir. lough, L. lacus (see lake (1)). The Loch Ness monster is first attested 1933.
remonstrance: c.1477, from M.Fr. remonstrance (15c.), from M.L. remonstrantia, from remonstrans, prp. of remonstrare "point out, show," from L. re-, intensive prefix, + monstrare "to show" (see monster).
dragon: early 13c., from O.Fr. dragon, from L. draconem (nom. draco) "serpent, dragon," from Gk. drakon (gen. drakontos) "serpent, seafish," from drak-, strong aorist stem of derkesthai "to see clearly." But perhaps the lit. sense is "the one with the (deadly) glance." The young are dragonets (14c.). Obsolete drake "dragon" is an older borrowing of the same word. Used in the Bible to translate Heb. tannin "a great sea-monster," and tan, a desert mammal now believed to be the jackal.
muster (v.): c.1300, "to display, reveal, appear," from O.Fr. mostrer (modern Fr. montrer), from L. monstrare "to show," from monstrum "omen, sign" (see monster). Noun meaning "act of gathering troops" is c.1400. To pass musters (1570s) originally meant "to undergo military review without censure." To muster out "gather to be discharged from military service" is 1834, Amer.Eng. To muster up in the fig. and transf. sense of "gather, summon, marshal" is from 1620s. Related: Mustered; mustering.
nixie: "water fairy," 1816 (introduced by Sir Walter Scott), from Ger. Nixie, from O.H.G. nihhussa "water sprite," fem. of nihhus, from P.Gmc. *nikwiz- (cf. O.N. nykr, O.E. nicor "water spirit, water monster," also used to gloss hippopotamus; Grendel's mother in "Beowulf" was a nicor), perhaps from PIE *nigw- (cf. Skt. nenkti "washes," Gk. nizo "I wash," O.Ir. nigid "washes").