Demons are very popular in modern belief and can be broken down in three categories.
Hell Demons: The "Actual" demons, as in the ones that are born from all the Hate and all evil in Hell, along with the Dark Fallen Angels.
Dimensional Demons: Beings that inhabit different dimensions or planes of existence. They could have dark intentions or be born out of spite, vile, and evil. These demons usually have an epheral existance or a not very solid composition in our plane of reality and the physical.
Earth's Demons: Beings that inhabit the Earth.
From the Greek 'daimon', or spirit, demons are powerful supernatural beings existing between god(s) and humans. Traditionally benevolent, daimon began to take on sinister themes within the writings of Plato. When the Old Testament was translated from Hebrew to Greek, the word mal'ak was used to describe foreign or evil supernatural beings. In the New Testament writings, most of which were written in Greek originally, the word daimon became associated with Satan and his minions and thusly took on a distinctly evil meaning.
The classical Greek daimon, Pan, was a nature spirit. The goat-like features of Pan have been adopted by Christian imagery in particular as those of demons. While Pan and other daimons gave their name to the Christian demons, their habits and nature as unpleasant beings of the underworld/death more resemble an amalgam of many other creatures from Mediterranean mythology, including the netherworld guards of Greece and Egypt.
http://en.wikipedia.org/wiki/List_of_demons
Demons walk the earth. They can take animal or human form. The numerous types and forms of demons have been listed below:
- Amon(Aamon) (Christian demonology)
- Abaddon (Christian demonology)
- Abalam (Christian demonology)
- Abraxas (Gnosticism)
- Abyzou (Jewish mythology)
- Ad-Dajjal (a.k.a. Dajjal) (Islamic eschatology)
- Adramelech (Assyrian mythology)
- Aeshma (Zoroastrianism)
- Agaliarept (Jewish mythology)
- Agares (Christian demonology)
- Agiel (Jewish mythology)
- Ahriman/Angra Mainyu (Zoroastrianism)
- Aim (Christian demonology)
- Akem Manah/Akoman/Akvan (Zoroastrianism)
- Ala (Slavic mythology)
- Alal (Chaldean mythology)
- Alastor (Christian demonology)
- Alloces (Christian demonology)
- Allu (Akkadian mythology)
- Amaymon (Christian demonology)
- Amdusias (Christian demonology)
- Ammut (Egyptian mythology)
- Anamalech (Assyrian mythology)
- Anzu (Sumerian mythology)
- Amy (Christian demonology)
- Andhaka (Hindu mythology)
- Andras (Christian demonology)
- Andrealphus (Christian demonology)
- Andromalius (Christian demonology)
- Antichrist (Christian demonology)
- Apep (a.k.a. Apophis) (Egyptian mythology)
- Apollyon (Christian demonology)
- Armaros (Christian demonology)
- Asag (Sumerian mythology)
- Asb'el (Jewish mythology)
- Asmodai/Asmodeus (Jewish folklore and Christian demonology)
- Astaroth (Christian demonology)
- Astarte (Semitic mythology)
- Asura (Hindu mythology)
- Azazel (Jewish demonology) and (Islamic mythology)
- Azi Dahaka/Dahak (Zoroastrianism)
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Baphomet (Demon, or image/idol thereof)
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Beherit (Phoenician mythology)
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Chupacabra (Latin American mythology)
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Davy Jones (sailor lore)
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Nyai Loro Kidul (a.k.a. Nyi Roro Kidul) (Javanese mythology)
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Orcus Roman Mythology
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Pontianak (a.k.a. Kuntilanak) (Indonesian demonology)
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Ravana (Hindu mythology - also seen by some Hindus as an aspect of the God Shiva)
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Salpsan, Satan's son (Apocryphal Gospel of Bartholomew)
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Tengu (Japanese Mythology Buddhist demon and Shinto spirit or god)
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Titivillus (Christian demonology)
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Wendigo (Native American Mythology)
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Zozo (Unknown Ouija Board Entity)
Therefore it might be best if you choose to think as demons in this four basic categories:
*Fallen Angels/Fallen Heavenly beings
*Dimensional (Exist in alternate universes/Realities)
*Demonic Monsters (Created by the devil, resoult of mans hate/desire/evil will, or black magic)
*Earthly monsters (Monsters/Mythical Creatures that might be classified as demons. Ex: Dragons, Ogres, Centaurs, Minotaurs, etc)
Please Note that these are not all as cut and dry as they may appear. For example "Evil dragons" could technically fall within all the categories, unless it is a specific type/category/specie of evil dragon, in which case it would fit much more nicely.