According to Unger's Biblical Demonology:
“It’s to be noted carefully that Hebrew possesses no precise equivalent for the Hellenistic Greek terms daimon or daimonion. [Furthermore] the rabbis divided demons into 2 classes: purely spiritual beings and half-spirits [halbgesiter].”
Yet, the LXX translates ’elilim in Ps 96.5 as demons.
Other Hebrew terms translated demons include:
Satyr (seirim, Lev 17.7; 2K 23.8; Isa 13:21; 34:14);
Shedhim, elohim (Deut 32.17);
And personal names like Gad (Isa 65.11);
And the only known female-demoness, Lilith (Isa 34).
You also have the rephaim, translated shades or dead ones who meet other nocturnal demonic creatures who inhabit desolate/destroyed places. These are described as animals like owls, jackals, hyenas, wolves, ostriches, vultures (Isa 13.19-22).
Emerson, in his book The Serpent's Seed (p. 71) notes:
"Unlike today, in ages past a common belief was that evil spirits [i.e., "demons"] were the ghosts of dead Nephilim/Rephaim."
The 1st book of Enoch [aka Book of the Watchers], written c. 300BC.
"The giants [Heb. nephilim] who are produced from [the cohabitation of] spirits [cp, spirits in prison, 1Pet 3.19] and flesh, shall be called evil spirits upon the earth, and on the earth shall be their dwelling; because they are born from men, and from the holy watchers [aka angels, Dan 4.13, 17, 23LXX] is their beginning and primal origin.”
"Here shall stand the angels who have connected themselves with women, and their spirits [children?] assuming many different forms are defiling mankind and shall lead them astray into sacrificing to demons as gods, (here shall they stand,) till the day of the great judgement in which they shall be judged till they are made an end of." [1 Enoch 15:8; 19:1-2]
This was the belief of the earliest Christian writers like Athenagorus (A.D. 137-175) who writes that some angels:
“fell into impure love of virgins, and were subjugated by the flesh. [Of these] were begotten those who are called giants. [And] the souls of the giants, which are the demons who wander about the world, perform actions similar [to fallen angels. They] haunt the air and the earth, and are no longer able to rise to heavenly things…”
Justin Martyr, 2nd Apology 5 (c. 200s AD).
Angels “transgressed the divine ordering, and by sinful intercourse with women produced offspring who are demons [cp. evil spirits in Enoch, Peter].”
The Almighty did not allow [the giants] to be brought back from death after they died, because they were of an evil seed. As a result they now wander and fight for control of people's bodies.
"Wicked demons [of the race of Titans or Giants] haunt the denser parts of the bodies. They also frequent unclean places on the earth. Since they are without bodies of earthly material….They also secretly enter the bodies of the more predatory, savage, and wicked of animals…."
