A possible fifth
Idea #5: The Toxic Waste dump theory. The idea that there are pools and pockets of power, magic or whatever that are going bad or seeping. Possibly left over from the Theocracy, maybe Gaetani god residue. It's either infecting the local plants, animals and people to make monsters or using them as prototypes.

I'm inclined to think it's a combination of influences, with #1 playing a major part.

The All Knowing Oracle...
...Charles pointed out Gaetani monsters are the children of the "evil" gods made because the "evil" gods were jealous of the "good" gods making humanity. So its their attempt at making humans.

Something Calvus would know because he's purchased copies or is borrowing copies of texts about Gaetani mythology.

And Chas would know
Well, that works too!

I like that.
I like that.

Mythologically true
Well, that's what is mythologically true, but mythologically true doesn't equal actually true, particularly in Gaetan. If Holy Church mythology is true (and it is! really, really true! Why, my seven times great grand mother met Reason herself!), then almost all Gaetani mythology is false.

Sure, I like that idea of the actuality that Calvus discovers (or the reflection fo the actuality that Calvus invents) aligning in some sense with the mythology, but beyond that, I have no strong feeling of which way is right. Personally, I kinda like all 5 theories proposed.

50,000 Gaetani can't be wrong
50,000 Gaetani can't be wrong.

Well, I know that Monkeys at
Well, I know that Monkeys at least are tolerated by monsters better than normal people (and CERTAINLY better than mages). I guess the Gaetani would view that as a sort of sibling truce?

I'm just pleased Reason is a
I'm just pleased Reason is a lady.

Ambiguously phrased.
I meant, "My seven times great grandmother herself met Reason."

Sadly, Holy Churchers are boringly sexist that way: Love is female and Reason is male.

Or filial truce
some monsters are the children of the Monkey (and others are the children of the Wolf).

I think individual Monkeys vi
I think individual Monkeys view themselves as the children of the Monkey, begat through the agency of human bodies. Or at least that's what the two halves of Ilba Perdix's Apprentice would tell you.

Whereas the monkey-brand monsters are the Monkey's progeny through non-human means, or they're the result of the Monkey tampering directly with a pre-existing being.

Just theorizing here...