A reader writes:

My husband and I were discussing the Gospel reading and he asked why demons aren’t mentioned in the OT when they’re all over the New.  Me:  huh?  They aren’t mentioned?  While frantically trying to remember my studies of Genesis and Exodus and not finding any demon references in them.  He said he’d tried to dig out references and there were hardly any from the OT.  Me:  maybe it grew it up in the inter-testamental period.  Him (dubious).  So… assuming he’s right, where does the concept of ‘demon’ come from that by NT times, everyone knows what they are?  Do you know as a former Bible only Christian, or can you or your readers points to some reference material on this?

The invaluable Catholic Encyclopedia comes to our help here:

In Scripture and in Catholic theology this word has come to mean much the same as devil and denotes one of the evil spirits or fallen angels. And in fact in some places in the New Testament where the Vulgate, in agreement with the Greek, has daemonium, our vernacular versions read devil. The precise distinction between the two terms in ecclesiastical usage may be seen in the phrase used in the decree of the Fourth Lateran Council: “Diabolus enim et alii daemones” (The devil and the other demons), i.e. all are demons, and the chief of the demons is called the devil. This distinction is observed in the Vulgate New Testament, where diabolus represents the Greek diabolos and in almost every instance refers to Satan himself, while his subordinate angels are described, in accordance with the Greek, as daemones or daemonia. This must not be taken, however, to indicate a difference of nature; for Satan is clearly included among the daemones in James 2:19 and in Luke 11:15-18.

But though the word demon is now practically restricted to this sinister sense, it was otherwise with the earlier usage of the Greek writers. The word, which is apparently derived from daio “to divide” or “apportion”, originally meant a divine being; it was occasionally applied to the higher gods and goddesses, but was more generally used to denote spiritual beings of a lower order coming between gods and men. For the most part these were beneficent beings, and their office was somewhat analogous to that of the angels in Christian theology. Thus the adjective eydaimon “happy”, properly meant one who was guided and guarded by a good demon. Some of these Greek demons, however, were evil and malignant. Hence we have the counterpart to eudamonia “happiness”, in kakodaimonia which denoted misfortune, or in its more original meaning, being under the possession of an evil demon. In the Greek of the New Testament and in the language of the early Fathers, the word was already restricted to the sinister sense, which was natural enough, now that even the higher gods of the Greeks had come to be regarded as devils.

We have a curious instance of the confusion caused by the ambiguity and variations in the meaning of the word, in the case of the celebrated “Daemon” of Socrates. This has been understood in a bad sense by some Christian writers who have made it a matter of reproach that the great Greek philosopher was accompanied and prompted by a demon. But, as Cardinal Manning clearly shows in his paper on the subject, the word here has a very different meaning. He points to the fact that both Plato and Xenophon use the form daimonion, which Cicero rightly renders as divinum aliguid, “something divine”. And after a close examination of the account of the matter given by Socrates himself in the reports transmitted by his disciples, he concludes that the promptings of the “Daemon” were the dictates of conscience, which is the voice of God.

It may be observed that a similar change and deterioration of meaning has taken place in the Iranian languages in the case of the word daeva. Etymologically this is identical with the Sanskrit deva, by which it is rendered in Neriosengh’s version of the Avesta. But whereas the devas of Indian theology are good and beneficent gods, the daevas of the Avesta are hateful spirits of evil.

So the quick and dirty answer to your question is that “demons” almost never get mentioned in the OT for the simple reason that the OT is in Hebrew and “demon” is a word that derives from Greek.  However, this is too simplistic.  As the history of “daemon” in Greek shows, the development of the concept takes time.  And this is true, not only for Greek speakers but for Jews as well.  The spiritual world, like the physical one, is full of mysteries for the ancients and they tend to tread lightly there unless their tradition makes something extremely clear (which it usually does not).  So just as the Greeks have a vague idea that there are good and bad “daemons” (that is, spiritual beings who involve themselves in human affairs), so the Jews have analogous ideas.  So while the Hebrews of Moses’ day believe in one God as the God of their fathers and the God of the covenant, it is not a slam dunk that they believe in one God in the sense that they think only one god exists.  instead, they often speak as though God is the great God, while the spiritual beings worshiped by other nations are not so much non-existent as inferior to him.  So the psalms say of God that he is “the great king over all other gods”.

Similarly, the entire story of the ten plagues in the Exodus is predicated on the idea that the God of Israel is making war on and defeating the gods of Egypt.  The plagues are calculated as affronts to and crushing defeats of various Egyptian deities.

The point is this: Israel is, in its own way, as conscious of the fact that there are a multiplicity of supernatural powers at work in the world as the Greeks and Romans are.  However, what the revelation of monotheism brings to the table (and eventually clarifies) is the understanding that all other supernatural beings are creatures of God and are either acting as his agents (in the case of angels) or are rebels against him and therefore his enemies and ours.  It does indeed appear that Israel only really starts to work this out in the intertestamental period (Baruch and Tobit are the only OT books that mention demons, and they are part of the Deuterocanon and are very late in OT history).  Possibly this is helped by exposure to Greek or Persian culture.  We don’t really know.  What we do know is that, by the time of the New Testament, Jews have, in large part embraced the idea of angels and demons (with the exception of Sadducees) and Jesus, of course, affirms this.  So Paul, significantly, regards pagan gods, not as non-existent, but as demonic:

Therefore, my beloved, shun the worship of idols. I speak as to sensible men; judge for yourselves what I say. The cup of blessing which we bless, is it not a participation in the blood of Christ? The bread which we break, is it not a participation in the body of Christ? Because there is one bread, we who are many are one body, for we all partake of the one bread. Consider the people of Israel; are not those who eat the sacrifices partners in the altar? What do I imply then? That food offered to idols is anything, or that an idol is anything? No, I imply that what pagans sacrifice they offer to demons and not to God. I do not want you to be partners with demons. You cannot drink the cup of the Lord and the cup of demons. You cannot partake of the table of the Lord and the table of demons. (1 Cor 10:14-21)

At the same time, Paul is also aware that God is at work in the hearts and minds of pagans (as, for instance, the discussion of Socrates’ “daemon” shows).  So he sees the Athenians’ veneration of the Unknown God as a sort of groping toward Christ. (Acts 17).  Similarly, the Christian tradition will, not at a theological level, but at a cultural one, make room for such holdovers of pagan notions as “being inspired by the Muse” (basically the image of a feminine spirit who moves you to write a good poem or song) and the idea is half-seriously connected with the actual doctrine of inspiration whereby the Holy Spirit actually inspires the utterance of true prophecy or the writing of Scripture.  So Paul himself, making easy and casual connection between the ideas of a biblical prophet and a Greek poet will refer to the Cretan poet Epimenides as a “prophet” when he writes to Titus (1:12).  Paul is not laying out a formal doctrine of the Inspiration of Epimenides and the Church is not failing to recognize inspired Scripture when it neglects to include the collected works of Epimenides in the Bible.  Rather, Paul is talking like a typical ancient Christian in seeing something spiritual going on in such works of human creativity that seem to connect us with a spiritual realm beyond ourselves.  What the nature of that connection is Paul doesn’t try to riddle out.  It’s one of those grey areas where mystery prevails.  But since, for Paul, the core of reality is God the Father, Son, and Holy Spirit, who creates all things, Paul is confident that whatever spiritual powers there may be in the world, ultimately they are either servants of Christ Jesus or his enemies.  If servants, then they like all things exist by, in, and for him.  If enemies, then God has already defeated them through Christ: “He disarmed the principalities and powers and made a public example of them, triumphing over them in him.” (Col 2:14).  Therefore, as he tells the Romans:

For I am sure that neither death, nor life, nor angels, nor principalities, nor things present, nor things to come, nor powers, 39 nor height, nor depth, nor anything else in all creation, will be able to separate us from the love of God in Christ Jesus our Lord. (Rom 8:38-29)