Lost city of Amdapor Hard [SPOILERS]

[Anonymoose]

But where did those reskinned sea horrors come from? Native life? Undead Machi pets?

In XI, the name was Sea Monk, because it came from the sea and had eight arms (with which to Hundred Fists you in the face).
In XIV, the name is Void Monk, I suppose because they come from the void and have eight arms (with which to Hundred Fists you in the face).

The Mhachi void mages used bound voidsent for all sorts of things, so I imagine you're correct that they were pets (after a fashion).