The Dark Divinity 3.0

[Berethos]

The best we can figure out given in-game context and Ferne's post is that all Eikons are primals but not all primals are Eikons (and even that's a stretch I wouldn't want to make yet); there isn't information enough to say much more than "Garleans call all Primals 'eikon'" and there exists some form of distinction between 'primal' and 'eikon' given Unuhalkai's dialogue, though it may be more superficial than some of us think

I think there's a couple possibilities here, admittedly among others. Either Unukalhai is misleading us (knowingly or unknowingly) with inaccurate information, though I'm not sure what he would stand to gain from that, or there's more to beings summoned by aether than our characters know, and it's to that which the little snake is referring.

Through the course of the MSQ we already learned that the Primals as we've seen them aren't truly creatures/deities that exist on their own but are a created image by those who summon them.

It's possible that the primals that we've faced so far, excluding Odin, are simply a subtype of summoned being, and that there were more powerful types (like Odin, that operated differently in terms of being summoned or sticking around) and that these are the Eikons that the Allagan empire faced, but still fall within the term "summoned beings" as referenced in Fernhalwes' post.

For me, the real question is what has led to Unukalhai making the distinction in the first place.

"Ah, but I speak not of the primals, with which you are exceedingly well acquainted. It may interest you to know that the term “eikon” and the beings to which it refers precede the Garlean Empire by eras. You see, it is the name by which the Allagans called godlike beings, the Dark Divinity Odin among them."

Notice that he doesn't say it's the name used by the Allagans to refer to summoned beings, just simply godlike beings, while also flat out saying he's not talking about the primals we've encountered so far. With Fernhalwes' post for context, it seems he's not 100% in his knowledge of these things, so it has me wondering why he believes they are different enough to be separate beings when according to the lore wrap up transcript they are at least in the same class of beings.