Hey! I spoke about pre-Islamic Arabian religion - both polytheism and monotheism - in an answer in this thread here that I'm going to pull from. I'd also recommend checking out /u/shlin28's answer in the same thread, as it talks much more about the Christian perspective and religion outside of the Arabian peninsula. I'm boarding a flight in a few hours, but if you have any follow up questions please ask and I'll try to get back to them as soon as possible!

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Pre-Islamic Arabian Polytheism

The religions of the Arabian Peninsula pre-Islam were a mix of Christianity, Judaism, and local polytheistic beliefs (idol worship, etc). The Arabs had a pantheon of gods, and there is evidence for the names of at least 100 of them in inscriptions, poems, and other writings. Unlike some other polytheistic religions, where one god controlled a specific aspect of nature (e.g: the rain, the sun, the winds) many of the Arabian gods instead had a specific realm of focus. One village might have their gods, and then there would be gods for an entire region, and then an even larger god, for example ʿAthtar in Southern Arabia.

People would give offerings to these gods, oftentimes milk or animals, in order to appease them or appeal to them. Lower gods could also intercede with higher gods (for example supplicating your village deity to intervene with ʿAthtar). The sheer number of deities gave rise to the old Arabic proverb "When you enter a village, swear by its gods". There was nothing wrong with accepting that another village was under the "jurisdiction", if we can call it that, of different gods.

Robert Hoyland in Arabia and the Arabs: From the Bronze Age to the Coming of Islam writes about the use of votive gifts and statues as the method of worship and dedication to these gods.

Just as crucial for execution of dedications to the gods were the votive gifts themselves. Very often these were perishable items (aromatics, foodstuffs, animals etc.), but there are two types that have survived in considerable numbers. One is the stone tablet or bronze plaque, on which is inscribed a person’s dedication and usually some expression of entreaty or thanksgiving. The other is the figurine or statuette (south Arabian/Aramaic s.lm, Arabic s.anam), usually human or animal in form (Pl. 28d). They are fashioned out of a variety of different substances, most frequently stone (e.g. steatite, talc, limestone), metal (bronze, silver or gold), alabaster, wood or clay. Numerous Arabian inscriptions contain the statement that so-and-so dedicates to such-and-such a deity a statuette in the hope of obtaining/in gratitude for receiving such-and- such a blessing.

Certain towns would host these pantheons. For example, the Kaʿba in Mecca, before being turned into the object of the Islamic ḥajj pilgrimage, was a shrine hosting over 360 gods and a common pilgrimage spot. In order to facilitate the pilgrims and the tithes that they brought, the ruling tribe of Mecca, the Quraysh, even enforced a calendar system in which 4 months of the year are considered ḥarām and during which no violence in permitted.

In addition to the pantheon of gods, there were various spirits and jinn, or Genies. These jinn later got adopted into the Islamic system of belief, becoming another form of created creature akin to but more powerful than humans. According to the Islamic tradition, the Jinn were created from smokeless fire and humans from mud, and although both were created with the intent of serving God, they also had the ability to choose.

Finally, there were magicians capable of working magic and charms. One could cast spells for the weather, to cure grief, or to divine the future. Some of the practices of these soothsayers were also retained post-Islam. One example would be al-Zajr, which is described by Toufiq Fahd in La Divination Arabe (qtd. in Signs, Omens, and Semiological Regimes in Early Islamic Texts from the Harvard Center on Hellenic Studies) as:

a method of divination which essentially consisted of throwing a stone at a bird and shouting at it. A bird which flew away to right of the thrower (zājir) was taken to be a favorable portent (tafāʾala bi-hi), while the leftward-flying bird was taken as an unfavorable one (taṭayyara). Testimony for the antiquity of this practice is found in a verse of Labīd, which also indicates that al-zajr was practiced by women. [46] This is supported by a ḥadī th related by one Umm Karz, who said: “I went before the Prophet and heard him say: ‘Let the birds stay in their places’ (var. ‘in their nests’).” Asked about the meaning of this ḥadī th, al-Shāfiʿī (d. 204/820) said: “The Arabs were masters at the technique of inducing birds to fly. When one of them wished to make a journey, he would exit his dwelling until he came upon some perching birds, which he would then cause to fly away. If they flew off to the right, he would continue on his journey; but if they flew off to the left, then he would go back home.”

The difference is that while the flight of the birds would have believed to be controlled by a diety pre-Islam (in this case, Awf, although I was never able to find too much information on him), they would have been controlled by Allah after the Revelation

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Pre-Islamic Arabian Monotheism

Not everyone was a polytheist though. In particular I want to highlight the existence of the ḥanīf. Being a ḥanīf essentially entailed in believing in Abrahamic monotheism (as opposed to the standard polytheism of the Arabian Peninsula at the time) but it wasn't a set religious creed. The Arabic root ḥ-n-f in ḥanīf means to incline, so ḥanīf was used in the Qurʾan to signal those who had returned to the monotheism of Abraham away from the idolatry of polytheism.

This sort of monotheism wasn't unknown. There were other figures around Muhammad's time who fell into such beliefs. One of them, the cousin of his first wife Khadija, Waraqah ibn Nawfal, was a hanīf with Christian leanings. Indeed, the fact that the Kaʿba (which became polytheistic pilgrimage center) was claimed to have been built by Abraham and his family meant that there would have been reasons for there to be ḥanīf around Mecca as a whole.

Wael Hallaq, one of the leading scholars on Islamic law, writes within his book The Origins and Evolution of Islamic Law that

Already in Mecca, Muḥammad conceived of himself as a ḥanīf, probably under the influence of a certain Zayd b. ʿAmr. Fundamentally monotheistic, ḥanīfiyya appears to have been a specifically Meccan religious development that was formed around the figure of Abraham and the Kaʿba, which he was believed to have constructed.

The ḥanīf definitely were partial to the other Abrahamic monotheistic faiths. Many of the ḥanīf ended up converting to Christianity (including Khadija's cousin Waraqah bin Nawfal) or, after the Revelation, to Islam. Zayd bin Amr was one of the few who remained fully ḥanīf, without converting to a more established religion.

Ḥanīfs as a whole were not a solifided movement, so they wouldn't have had a clergy. Certain men would preach in the countryside to those who would listen, as Muhammad would do later after the Revelation. Prior to this point, Muhammad would listen to Zayd bin Amr preach, which probably influenced the development of his own branch of monotheism.

Despite not being a solidified movement, the ḥanīfs tended to have some unifying characteristics. In an old paper, Charles Lyall wrote (by which I mean 1903, so methodology has definitely changed) about 3 of these characteristics. The Ḥanīf

all belonged to the Ḥijāz and the West of the Arabian Peninsula; (2) ... their doctrine was distinct from Christianity, although several of those who professed the Ḥanīfite faith adopted that religion, and was also distinct from Islām; and (3) ... it had certain specific features - rejection of idolatry, abstention from certain kinds of food, and the worship of "the God of Abraham"; ascetic practices, such as the wearing of sackcloth, are also ascribed to some of the Ḥanīfs (773 - 774).

The ḥanīf are not mentioned in any of the verses concerning the ʿaqd al-dhimma, or contract of protection, that was extended by Muhammad to the People of the Book. The traditional scope of the people of the book were the Jews and the Christians, the two other dominant monotheistic faiths who possessed certain customs, traditions, etc. (Although the definitions gradually expanded, with Zoroastrians, Mandeans, and controversially at times Hindus being included).

Part of this could very well be that there simply weren't many ḥanīfites, and that they were not organized enough to be considered a group worth mentioning. Charles Lyall writes that the word ḥanīf was only used by 2 poets in the time of Muhammad, and they attributed the adjective to only 10 men total (this is when ḥanīf is used to refer to a non-Islamic monotheist. The word can also be paired with the word Muslim to refer to a righteous Muslim). Of course, these are only the men important enough to be mentioned, but it holds that they were often individuals and not entire tribes who held ḥanīfite beliefs, so they wouldn't warrant the same official protections.