You might have noticed a few Greek gods receiving cameos in Prince Caspian. Bacchus (a.k.a. Dionysus) gets the largest role, and he's still followed by his right-hand bro Silenus. Peter mentions that he once chilled with Pomona (2.35). And the list of lesser deities popping up in Narnia includes satyrs, fauns, dryads, nymphs, maenads, and hamadryads. Someone's been reading up on their Shmoop mythology.
But wait a second? Isn't the Narnia series usually considered to be a Christian allegory? And wouldn't many Christian denominations consider these to be pagan gods? (See below for more on that word.) So why does Lewis, a famous Christian convert, not only include non-Christian deities in the story but also side them with Aslan, Jesus' counterpart in the story?
According to Clotilde Morhan, Lewis believed that:
[…] the pagans were more spiritually enlightened than most of his contemporaries. To the pagans there was more to reality than the material world. They saw something modern man is blind to. Spirit and matter were indivisible to them. They looked at the sun and they saw, if not a god, at least an expression of the divinity. (Source)
Sure, Lewis didn't agree with the pagan answer to the question, but he did like the idea of a spiritual world living in tandem with the material one.
And boy do we see that in Prince Caspian. You'll notice that all the non-Aslan deities are gods connected to material things in nature: trees (dryads and hamadryads), rivers (nymphs and the river gods), and fruit (Pomona). Bacchus may be the god of winemaking, frivolity, and ecstasy, but he's the head honcho of the wine harvest, too, so he's hardly a stranger to nature.
So the inclusion of the other gods might symbolize the desire to promote something more than material. And the gods of Greek myth were already well known and available to work, so, hey, why not give 'em an invite?
But that's not the only way to read the symbolic use of other deities hanging out in Narnia. Here are two more:
(1) We traditionally think of the Judeo-Christian religions as being monotheistic (that is, they believe in one god only), but passages in the Bible suggest that this wasn't always the case. For example, Psalm 82:1-6 states, "God standeth in the congregation of the mighty, he judgeth among the gods." And Jeremiah 10:11 states, "[t]he gods that have not made the heavens and earth, even they shall perish from the earth, and from under these heavens." In both cases, the Judeo-Christian god is clearly not the only god, but he is the most powerful amongst those gods. This kind of stuff is all over the Hebrew Bible.
In Prince Caspian, like in the Biblical passages, Aslan isn't the only god—but he is their leader. During Aslan's invasion of Telmarine territory, "Aslan [is] leading, Bacchus and his Maenads [are] leaping, rushing, and turning somersaults, the beasts frisking round them, and Silenus and his donkey [are] bringing up the rear" (14.47). And it is Aslan who commands Bacchus to "'[d]eliver the [rivergod] from his chains" (14.52). So it's possible the novel is tapping into earlier views of the Judeo-Christian god in constructing Narnia's own religious mythology.
(2) If you want to get all psychological about it, you could connect Bacchus to the concept of the Dionysian (i.e. ecstasy), and Aslan might represent the opposite, the Apollonian (i.e. reason). In the scene where Susan and Lucy first encounter Bacchus, Susan notes, "'I wouldn't have felt safe with Bacchus and all his wild girls if we'd met them without Aslan'" (11.65). Translation: reason protects people from wild abandon and ecstasy.
Of course, these are just a few potential readings for why Prince Caspian includes so many Greek deity shout-outs. Feel free to rework them or head in a completely different direction when tackling these parts of the book yourself. We won't mind if you do.
Definitions have a tendency to be a bit, let's say, slippery. For example, we used the term pagan above to describe a possible way to consider the Greek gods because it's probably the way they would have been described in Lewis's day. In that context, the word is an uncomplimentary term used in Judeo-Christian tradition to refer to the gods and customs that are not their own—especially the ones associated with nature worship.
But we should point out that today many religions—including Wiccan and Druidic—use the terms pagan and paganism to describe their own beliefs, taking a traditionally debasing term and turning it into something positive. If you're interested, you can learn about the history and uses of the term here.