My suspicion, mostly unfounded, is that it's a proto-form of the Horned Rat. I considered it because it's the most interesting narrative possibility.
But here's some actual evidence though. First, one his symbols is antlers, and he's often depicted with them. He's also animal associated. On the other hand, we have the Horned Rat. The horn part is pretty important for it, so it makes sense it would show up. And if it somehow turned the city of Tylos into Skaven, perhaps they turned themselves into one as well.