Looking at the art and abilities, I think i know what it's supposed to be, flavor wise.
To me, it looks like the phyrexians took a Plated Slagwurm, compleated it, and modified it to accept a pilot. When installed, the merged intelligence and digestive track grant a sense of strategy and the capacity to feed on prey.
What's more, the modifications are so extensive that, if the "second brain" is killed, and no other creature is around to jump in, it can retreat and regrow it's second brain, and get back in the fight.

That had to be what they (the design team) were thinking. Beyond that, i got nothing.