Some ideas based on earliest English uses (all from the OED):
Orc: Either a unique monster able to eat nearly the entire world or a multi-headed monster descended of incubi.
1598: Insatiate Orque, that even at one repast Almost all Creatures in the World would waste. 1656: Who at one Stroak didst pare away three Heads from off the shoulders of an Orke, begotten by an Incubus.
Ogre: A demon who devours humans.
1713: He perceiv'd that the Lady..was a Hogress, Wife to one of those Savage Demons call'd Hogres, who stay in remote places, and make use of a thousand wiles to surprize and devour Passengers.
Goblin: A mischeivous and ugly demon or fairy.
1327: Sathanas..Seyde on is sawe Gobelyn made is gerner Of gromene mawe. 1500: Many manyeres of thinges, the whiche somme called Gobelyns, the other ffayrees, and the other ‘bonnes dames’ or good ladyes.
Hobgoblin: A mischievous imp or sprite.
1530: Hobgoblyng, goblin mavffe
Troll: Apparently a type of undead.
1616: The said Catherine for airt and pairt of witchcraft and sorcerie, in hanting and seeing the Trollis ryse out of the kyrk yeard of Hildiswick.
Bugbear: A creature that eats children but cannot stand up to grown men.
1581: Meare bugge-beares to scare boyes. 1607: Certain Lamiæ..which like Bug-bears would eat up crying boys.
Ettin: Any giant
1205: Heo funden ithon londe twenti eotandes... Hit hatte there Eotinde King.
Ettercap: Any spider or ill-tempered person.
1000: With attorcoppan bite. 1725: Tis dafter like to thole An ether-cap like him to blaw the coal!
Kobold: A spirit that haunts mines
1635: The Parts Septentrionall are with these Sp'ryts Much haunted..About the places where they dig for Oare. The Greekes and Germans call them Cobali.
Gorgon: The species to which Medusa belonged
(not from the OED, well-known to students of Greek mythology)
Mephit: A bad smell
1636: What comes of all your cates? the jakes can tell, Which turnes your gold into Mephitis smell.
Sprite: any spirit
1375: As he hyr this had talde, the spret vn to god scho yald
Troglodyte: Any cave-dweller
1555: The Troglodites myne them selues caues in the grounde, wherin to dwell.
Wight: Any living being
Beowulf: Wiht unhælo, grim and grædigh
Worg: From Tolkein's "Warg", a large and evil wolf
1937: Even the wild Wargs (for so the evil wolves over the Edge of the Wild were named) cannot climb trees.
Phantom: Illusion, unreal thing
1300: Hit neys bot fantum [v.rr. fanton, fantom] for to say, To day it is, to moru away.