As others have started to say, Judaism isn't really the right word to describe that religion. Judaism is a monotheistic religion. It isn't a continuation of what the previous Canaanite religion was, it represents a unique development in history and religious thought.

The type of religion that Yahweh-worship probably came from would not have been a polytheism like the Greeks or Egyptians had. There wasn't a group of deities who each had different roles and powers in the cosmic economy. It had a more familial arrangement. There was one high God El or El Elyon, who was responsible for creation and represented some kind of abstract, ultimate power, and he would have had a consort. El had sons, and each son was the patron-God of an allotted people. Yahweh would have been the God of the Hebrews. Other gods existed, but Hebrews had no business worshipping them. And Yahweh would have also had his own consort. People have also theorized that there might have been a personified Wisdom worshipped at the First Temple.

A good source for this is The Origins of Biblical Monotheism.

How monotheism eventually won out is a much more complicated and speculative question.

As an Israeli Tanakh (OT) teacher, I'll do my best here, excuse my English: First, "Judaism" was never Polytheistic, nor Henotheistic. The Israelite religion of the Biblical Hebrews as we know it from the OT, has gone through few stages on its way to become the Judaism we know today, which is Monotheistic, which means - denies completely the idea of other gods besides Yahweh. Now, the OT itself never stop to remind us that the Israelites continue to worship other gods besides Yahweh: there simply too many places where Moses, Joshuah, the Judges, and almost all the prophets demand Israel to follow Yahweh and to stop being "polytheistic" but it's clear from all those speeches which go through the entire history of Ancient Israel in the OT, that they worshiped both Yahweh and the other gods at the same time. One interesting example is from Jeremiah 44:16-18, when Jeremiah is mad with the Jewish refugees from Jerusalem and demand them to worship Yahweh, but they answer him shamelessly: As for the word that you have spoken to us in the name of the Lord, we will not listen to you. 17 But we will do everything that we have vowed, make offerings to the queen of heaven and pour out drink offerings to her, as we did, both we and our fathers, our kings and our officials, in the cities of Judah and in the streets of Jerusalem. For then we had plenty of food, and prospered, and saw no disaster. 18 But since we left off making offerings to the queen of heaven and pouring out drink offerings to her, we have lacked everything and have been consumed by the sword and by famine.” - Which means, for them, worshiping the queen of heaven (probably, Ashtoret, or in her more known name: Ishtar, the Babylonian god) - is their real tradition. So we do know that the Israelites worshiped many other gods: Ishtar (Ashtoret), Baal, El and so on. What we also know is that those gods are much older than the Israelite people. We found in the city of Ugarit, northern to Canaan, mythological stories about El, Baal, Anat, Yam, Mot and Levithan, Tanin and other gods and monsters. These are mentioned in the OT, but with the Henotheistic Agenda: Yahweh created the Taninim (the Canaanite sea monsters, as we found out in Ugarit) on the third the day of creation. Yahweh splits the sea to two in Exodus 15, and the word for sea in Hebrew is Yam, like the name of the sea god in Ugarit. Yahweh can revive the dead in the OT, defying Mot, the god of the dead in Ugarit. We have only few short sentences in the OT that testify about an ancient layer of polytheistic mythology about Yahweh the warrior who defeats the sea monsters (Job 26:12-13; 7:12; Psalms 74:13), Yahweh as a god-king in pantheon of gods (Psalms 29), Yahweh as a southern god who comes from the desert (Deuteronomy 33:2-3) and so on. Outside the Biblical text we have found so far only few inscriptions in ancient Hebrew from the late of 9th century BCE, in a site in Sinai Desert, called Kuntillet Ajrud. One of the inscription says: Yahweh of Samaria and his Asherah, and this is an amazing proof that Yahweh has been worshiped with female-god. We understand Asherah is female because of the drawings under the inscription. Because of this, I would say that Asherah is the only god I would consider as Israelite, or Jewish god, besides Yahweh. About the second part of your answer, the transition was definitely completed through the late books of the OT, when Yahweh is considered as universal god, and the other gods are simply symbols of human hubris and nothing more: second Isaiah, Daniel, Malachi etc. In those late books, the mythological layer we see in Genesis, Exodus, Psalms and Job does no longer exist.

I'm Jewish and I don't think I've ever heard of polytheistic Judaism before. Can you please describe what specifically polytheistic Judaism entails?

I think you mean henotheistic or monolatrous, yeah?

I think that part of the issue in getting answers that conform with sub rules is the way you phrased the initial question, OP: Judaism is both a religion and an ethnicity. I would say that if you don't get some good answers, you might consider rephrasing it as something like "What do we know about the other gods in the region of Palestine/Israel/Canaan/Wherever during the rise of Judaism?"

Here I am referring to Canaanite deities. These were beliefs of the peoples of Canaan, but not necessarily Jewish peoples, though much of their populations came to be conquered and assimilated by Israel. The challenges in this mythology lay in a theological practice- the actual names of deities were never meant so be spoken (similar to the secret name of YHWH). So in many instances, we just have deities referred to as Ba'al - Lord- Ba'alat- lady. That was a title, not a name.

Thus there was Ba'al Tsaphon, Lord of the North; Ba'al Shamim- lord of the skies (also possibly considered El, or YHWH). Besides Asherat/Ashtart/Qedesha (the Mother of Heaven), there was also Eshmum/Adoni/Adonai- the Son of God, God of the Plants and Vegetation, who like Tammuz, died and was revived.

Sources: The Cambridge Ancient History Vol 2. Part 2, miscellaneous authors; New Larousse Encyclopedia of Mythology

I haven't heard anything like this before. Do you have any sources?