examples:
Death
the personification of death
Gargantua
a voracious giant in Francois Rabelais' book of the same name
Maxwell's demon
an imaginary creature that controls a small hole in a partition that divides a chamber filled with gas into two parts and allows fast molecules to move in one direction and slow molecules to move in the other direction through the hole; this would result in one part of the container becoming warmer and the other cooler, thus decreasing entropy and violating the second law of thermodynamics
Martian
imaginary people who live on the planet Mars
Humpty Dumpty
an egg-shaped character in a nursery rhyme who fell off a wall and could not be put back together again (late 17th century)
Jack Frost
a personification of frost or winter weather
Mammon
(New Testament) a personification of wealth and avarice as an evil spirit
Father Christmas
the legendary patron saint of children; an imaginary being who is thought to bring presents to children at Christmas
Tom Thumb
an imaginary hero of English folklore who was no taller than his father's thumb
types:
giant
an imaginary figure of superhuman size and strength; appears in folklore and fairy tales
hobbit
an imaginary being similar to a person but smaller and with hairy feet; invented by J.R.R. Tolkien
mermaid
half woman and half fish; lives in the sea
merman
half man and half fish; lives in the sea
Cadmus
(Greek mythology) the brother of Europa and traditional founder of Thebes in Boeotia
monster
an imaginary creature usually having various human and animal parts
witch
a being (usually female) imagined to have special powers derived from the devil
sylph
an elemental being believed to inhabit the air
unicorn
an imaginary creature represented as a white horse with a long horn growing from its forehead
halcyon
a mythical bird said to breed at the time of the winter solstice in a nest floating on the sea and to have the power of calming the winds and waves
Argus
(Greek mythology) a giant with 100 eyes; was guardian of the heifer Io and was slain by Hermes
Cyclops
(Greek mythology) one of a race of giants having a single eye in the middle of their forehead
ogre
(folklore) a giant who likes to eat human beings
phoenix
a legendary Arabian bird said to periodically burn itself to death and emerge from the ashes as a new phoenix; according to most versions only one phoenix lived at a time and it renewed itself every 500 years
houri
(Islam) one of the dark-eyed virgins of perfect beauty believed to live with the blessed in Paradise
Jotun,
Jotunn
(Norse mythology) one of a race of giants often in conflict with the Aesir
Mimir
(Norse mythology) giant who lives in the roots of Yggdrasil and guards the well of wisdom
Amazon
(Greek mythology) one of a nation of women warriors of Scythia (who burned off the right breast in order to use a bow and arrow more effectively)
Gog and Magog
biblical names of the enemies of God's people who wage war against God at the end of the world
golem
(Jewish folklore) an artificially created human being that is given life by supernatural means
hero
(classical mythology) a being of great strength and courage celebrated for bold exploits; often the offspring of a mortal and a god