User blog comment:LEONIDAS 666/The Unforgive Arch Angels/@comment-156.108.20.68-20120720193336/@comment-5131965-20120907124355

no its either Magnus, or a grey knight, the Emperor did not have enough time during the Horus Heresy to fight a titan, he was replaced on the Golden throne by Malchaidor so he could teleport up to the Vengeful Spirit.

Another person who could have done it (not a daemon, they prefer corruption) is Sanguinius, he had the perfect resemblance of the Emperor, and all his traits, although he was much younger, had wings, and here's the big bit...

He was blonde for almost 200 years after the radiation on Baal bleached his hair blonde. by the Horus Heresy his hair colour was Black.

the part about sanguinius's hair colour I got from studying the effects of bleaching in science class, and all the pictures depicting him blonde prior to the Horus Heresy.