The story: The sinister Willhaam Gaatz was the lead software engineer on the IG project team that produced IG-88 (and the hardly-known IG-87). On the fateful day that IG-88 was brought online, and then subsequently obliterated everyone in sight, the good Mr. Gaatz was at home "ill". In truth, he had become disgruntled with the direction the project was headed in, and he had tweaked the program in such a way that the IG droid(s) would go nuts when brought on line. For awhile after that day he hid out and worked on his own IG droid, which he wanted to build on a chassis similar to a protocol droid. This one would go out and pirate Gaatz into a rich lifestyle - without much risk for Gaatz. IG-91 is more mobile than IG-88 was, and he has internal gas mechanisms that allow him to gas subjects through a mouth nozzle. The first task for the droid was to steal a Fargo-class armored light transport (an armored car), which they then together modified into the Plunderer. IG-91's main function is piracy, although he has taken on the rare bounty hunter job.
The MOC: first, I was going to with the name IG-90, but I saw minifig on FBTB with that name, so I deferred to IG-91. The main torso on IG-91 came from a Doc Ock, and the torso from Gaatz came from a T-16 minifig.