In Pratchett's Discworld:-Edit
The concept of the Rogi, as such, does not exist. however, firmly embedded in the Discworld is the idea that everything must have its opposite. Therefore if Igors (and Igorinas) exist, the Discworld must also have its Rogis and Anirogis.
In the Pessimal Discworld:-Edit
the Rogi is an anti-Igor. Hewn from the same material and very possibly an ofshoot of the same clan, the Rogi will know all there is to know about dismantling people, often in inventive and exquisitely painful ways. They just aren't as up to speed on re-assembly.
The "Igor" at undead bar Biers may be a Rogi.