Luperci
The actual de jure Honour Guards of Horus Lupercal, NOT the Justaerin. The Luperci, ("Brothers of the Wolf" in Cthonian), was a special formation of the Sons of Horus during the Horus Heresy.
Overview[edit]
Formed with the assistance of the Word Bearers, the Luperci were for all intents and purposes, essentially the Sons of Horus version of the Gal Vorbak Possessed Chaos Space Marines. The Luperci were formed by Maloghurst after the Drop Site Massacre and were first employed during the Battle of Molech.
Now, although Horus never officially had an Honour Guard formation for himself. He de facto had multiple, with the Luperci being the second generation of Honour Guards. Now, questions on which is the 'official' Honour Guard for the Sons of Horus has been a confusing mess, which is not helped given that Horus himself treated both of them equally the same thing. All we do know is that Horus at that time, liked to roll around in multiple Secret Service agents as he was going to town on coo-coo crazy land and was getting increasingly paranoid from even his own allies. So more bodyguards never really hurt.
As far as equipment goes, whilst we aren't sure what the average Luperci carried around as weapons since, as Possessed, they ARE the weapon. We do know that nearly all of them wore Mark IV: Maximus Armour. Contrast this to the more sluggish Justaerin, and the Luperci formed the Honour Guards that can keep up with Horus' blinding speed.