I believe that Placidusax is/was a true Rebis, an alchemical melding of the White Queen and the Red King... and then some.

In Zullie the Witch's examination of Placidusax, they call out the fact that Placy once had five heads, and that one of his remaining heads is female

Next is Zullie's look at Bayle and the fact that 2 of Placidusax's heads are still bitten down into Bayle's flesh and hanging off of him. I don't recall if those heads are gendered in the files.

This is known and uncontroversial

What I put forward that is raw speculation:

Placidusax was a grafting of five dragons likely the FIRST ever living grafting, one being the god that would eventually 'was fled' That head being the missing one between what Placid has now and the two attached to Bayle. This is the template for the Rebis that Marika was trying to recapitulate with Radagon and why when we meet them they share a single body

The god 'fled' in the battle with Bayle, likely being torn off as well but not having bitten down was lost to the ages. Likely dead as whatever snake Marika stole her initial Grace from.

There is some evidence that Farum Azula technology was not made with forges, and the way that dragons and the honored dead are embedded into solid rock implies that Dragon culture technology manipulated matter directly to shape stone and metal into structures and tools, and that dead things being embedded would act as some form of protection for 'their lord'. This feels like a precursor to grafting that I will call dragonmelding. Dragonmelding is the source of all the structures and weapons originating in Farum Azula. I am not sure exactly what the mechanism is but the Cleaver and the ruins greatsword are implied to have been made without fire or tools.

Since those ancient dragons were 99% superock then melding would work with them if as it seems it only works on nonliving matter, I wonder if that means under Placidy's rocky surface there are 5 dragons all stacked on top of each other a-la Pinwheel

I believe there is some connection between dragonmelding and grafting, and that they both might be gifts of their various Greater Will's Children patrons, as a requirement to contain the Elden Ring. Maybe since the Elden Beast seems inorganic and Metyr is excessively organic, the clients of the Elden Beast (beastmen, dragons) became more rocky, while the clients of Metyr (trees) became more fleshy

MAYBE FLESH IN ELDEN RING ONLY EXISTS BECAUSE OF METYR!

In any case, TL;DR: Placidusax's 'god who fled' is one of the heads he lost in the battle with Bayle, and may be the first ever grafted being as an 'even truer Rebis'