-Avernii, as has been previously stated, was Ferrus' favoured, the company of First Captain Santar (Rust in Peace) and had the highest proportion of Morlock veterans. (are Morlocks even still a thing?) They also got first access to Gorgon plate, and were largely wiped out on Istvaan compared to the other clans who took proportionally less casualties.
-Raukaan was the star of the largely-beloved novel 'Wrath of Iron' in the 40k timeline, and the...much less beloved, shall we say, 'Clan Raukaan' supplement in 6th edition. There doesn't seem to have been anything particularly noticeable about them during the Heresy however.
-Sorrogol, Felg, Gaarsak, Ungavarr, Vurgaan, and Kaargul are confirmed to exist in the Heresy, but have relatively little fluff concerning them otherwise. It may be as stated prior that Ungavarr had the most fond memories for the Storm Walker days, and Meduson was definitely a Clan-Captain of Sorrogol.
-Haarmek, Borrgos, Morlaag and Dorrvok are extra clan-companies mentioned as active in the post-Gathering Storm timeline/8th edition codex. (with Dorrvok being the Scout company) None of these have been mentioned to my knowledge during the Heresy however.
-Clan Atraxii has the same logo as Clan Atropos, one of the three clans of the Sons of Medusa chapter that forms in the M35 AdMech-wide Moirae Schism. The other two are Lachesis and Mageara. (No known relationship to the Knight class to my knowledge) There isn't any major fluff on these clans specifically from the Heresy, but given the nature of Medusa and the Moirae schism itself, it's possible these guys had a predilection to Techno-Shamanism, soothsaying and the like even at an early stage.
-Last but certainly not least of the 'major' clans is the one I play, Clan Morragul. Morragul was led by Autek Mor, a Terran who had supposedly had some funky gene-seed stuff done to him, and was where the 'outcasts and most unstable' members of the Legion were put by Ferrus. They had a Grand Cruiser called the Red Talon, (wink wink nudge nudge the surname of the Red Talons Marine shown in the 8th ed book is literally 'Mor') wore panels on their armour such as pauldrons, favoured aerial and sub-orbital attacks, and I think they had a higher-than-average proportion of Meduson Immortals but can't find a source for that. They're also Absolute Madmen who get up to shenanigans in 'Retribution' from Forge World.
Finally, there's Clan Brannsar, which was/is the OC company of Iron Hands Fanatic on these boards. The name is currently used to represent a bundle on the Forge World website however, which...makes them canon, I guess?
Essentially, the thing to take away is that most of these guys have very little Heresy fluff beyond a name, and after Istvaan the Clan structure obviously begins to break down anyway - at least to a degree. The only ones who have 'immutable' things known about them at the time are Avernii, Morragul, Sorrogol as Meduson's clan, and possibly Ungavaar. I do like the 'head-canon' of Atraxii being into some weird stuff themselves, but that's all it is - head-canon. Source for all of these is Book 2: Massacre, Book 6: Retribution, Shattered Legions, Feat of Iron, Fulgrim, The Badab War - Volume 2, and Codex: Space Marines (8th edition)
Edited by Gabriel Santar, 06 July 2018 - 09:19 AM.