I am happy to announce that after nearly a year of searching on and off, I finally solved the mystery of Geshniz's Metal Skeleton.
It is a special summon from a Black Ring Summoner in the room where you first fight General Stone. Oddly enough the behavior is quite unique in that it seems to banish itself after a few seconds so you have to kill it as soon as possible.
That is why no one has it in their logs.
