A form taken by Molag Bal, accourding to Vivec's Sermons. Whatever that means.
The Ruddy Man was a god/chieftan of the dreughs, who was
a form of Molag Bal, but not his
original form. I believe Sermon 28 sheds a little light on this relationship and alludes to the role of a dreugh civilization. Basically yes, the dreughs are intelligent, they were civilized, and currently completely hostile in most (if not all) TES games.
EDIT: After reading Sermon 28 again, it seems like the Ruddy Man was nothing more than a set of Dreugh-skin armor possessed by Molag Bal. This skin, called the Ruddy Man, would be worn by a mortal (first by a Velothi child, then by a shaman of the House of Troubles) and then the mortal would be transformed physically by the skin into a dreugh-like monster chieftan. Molag Bal simply possessed the skin and acted through it (later leaving it after its second appearance with the shaman).