From what I understand, the Eye was on multiple primarches armour, such as Horus and Sanguinious. the symbol was know as the Eye of Terra and represented that the Primarches and there legions were the Emperors eyes while he was away. The Eye of Terra was not know as the Eye of Horus until Horus turned to Chaos, and used the Eye of Terra as his personal marking, Therefore making the Eye of Terra an evil symbol of Chaos and from then on known as the Eye of Horus.
Hope that wasnt too confusing. lol
"Blood cries out to blood they say, so let us spill their blood so we can hear it all the clearer."