Horus' story was a sad one IMO, he was once such a great man...and now look at him; that head used to have brown hair, he used to be dashing and handsome and now he is an ugly bald dude with some serious plug fetish. His armour used to be white and glamorous...and now it is as black as the abyss and covered in heads and skulls...the only thing the same as the old him is the coat of wolf fur on his back (though now it has skulls n stuff in it)
no he wasn't, they never ACTUALLY say what he looks like in the HH books and besides even the first HH book IS in the later days of the Great Crusade, he has just become Warmaster; there is an entire era where he; fought beside his father in his youth, then led his own legion, then led multiple legions because they hadn't re-united with their own Primarchs yet. also there are multiple images as well as a portrait of him with short brown hair. also if you look closely at the cover of HH book "The Primarchs" you will see that he isn't entirely bald, he has dark brown hair at the back but not at the front (kind of like a stereotype monk). the only Primarch that were always bald were Ferrus Manus and Perturabo I think, even Angron had very short hair at one point. here is a portrait of Horus when he was young noldofinve.deviantart.com/art/…
.....yeah, now I remember that little info, there had been many theories and hints of the Blood Ravens being descendent from the Thousands Suns Legion, the biggest hint is their abnormal amount of Psykers and Librarians...their affinity to tap into the Warp energies.
so yeah there is a chance that Magnus indeed is their Primarch.
however there are other 2 Primarch that are lost as well, I think they had dragged this...mystery long enough.
the 2nd Primarch and the 11th Primarch if my memory serves correct.
even the Primarchs themselves are hesitant to talk about those 2...so something really big must have happened to them.
GW said they left those two without legions or Primarchs so fans could create their own legions/chapters. Since half of the legions sided with the Emperor, and half to the Warmaster, they had two legions be left blank.