Giant rat ogres don't fit into any specific army list category. Depending on the base size, they could be used as as mutant rat ogres or Dogs of War giants (a rare choice in a Skaven army).

This guy I could not be happier with. Took tons of work, but he looks great. It's the entire Necromunda Caller miniature for starters, which is not cheap. The legs and tail need complete repositioning to stand upright. To get the tail to bend backward, I had to cut it in three places. Then the gaps in the hips and torso needed filling, and they were BAD, maybe the worst I've ever seen in a model. The left arm is a rat ogre arm, the right is from an Inquisitor model. The head also needed so much repositioning that I had to add a significant amount of additional cervical spine. Wow! It stands as tall as the old metal giant. See the size comparison photo at the bottom.

By this time, I had figured out that I was going to do some giant rat ogres for Dogs of War giants. At first, I didn't know whether to try to keep them more in scale with the other rat ogres, but why limit myself? I had some bitz from one of the Giants of Albion - painted, as you can see - I was too lazy to strip it, figuring that it will mostly get sculpted over. To get a decent pose to the legs, I had to completely resculpt the hips. To get the broader shoulders and narrow waist of the rat ogres, I'll completely resculpt the torso, and probably make it mostly hairy so as not to resemble the giant from whence it came.

What's next:
I've got grand plans for giant rat ogres, including something based off the GW plastic giant, and maybe an Eshin giant rat based off the Forge World model. These are low priority, and lots of work, so don't expect anything very soon!