This guy started out as a Tech-Adept or Magos of some kind. At the moment he seems fairly un-augmented, but that will change once the paint starts to go onto the model. Of course, the giant mechanical insect/arachnid that he's using as a palanquin keeps things fairly interesting. I've seen quite a few excellent Mechanicus/Spider centaur/Drider-type models, and was tempted to go down that path, but hopefully I've created something a little more unique to my army.
This one started off planning to be THE arch Magos in my army, but he has kind of grown (and at the same time diminished) into some kind of massive Praetorian monstrosity.
The basis for his tracked unit is a FW Grot tank, clad in a layer of plasticard to tidy things up a little. The autocannons are from the Aegis Defence Line quad gun. I'll mount this guy on a large, oval base and perhaps he could be used as a Forgefiend.