Join Date: Sep 2010
Location: In the shadows ...
To my understanding originally Horus had the "Eye of Terra", the ever watchful eye of the Imperial homeworld.
The red eye you are seeing is the Eye of Horus. Its still used, in different designs, by the Black Legion in the 41k. Its one of Horus' ways of showing that he was the True Master of Mankind.
The chaos gods abandoned Horus most likely because they saw the can of whoop ass coming their way and wanted out of the way so as not to get fucked up!