Breacher Siege Squad
A type of Legiones Astartes unit used during both the Great Crusade and Horus Heresy, Breacher Siege Squads were, as the name suggests, a specialized unit of Legion Tactical Marines geared up for sieges and fights within Zone Mortalis areas, though they were also used in void-warfare (especially when it came to ship boarding). Essentially, these guys were trained exclusively to fight in the most hazardous places the battlefield had to offer.
Clad in Void-Hardened versions of the MK.III Iron Armour, as well as armed with Boarding Shields and an array of assault and siege weapons, Breachers had better chances of survival and could reliably achieve their objectives (though considering how terrifyingly excruciating places like Zone Mortalis were even for the then young Legions, it says a lot). Some Legions (like the World Eaters, Imperial Fists, Salamanders, Iron Warriors, and Iron Hands being the most prominent ones) were geared up specifically for Zone Mortalis battlefields and had large numbers of these specialists.
In the modern, Post-Heresy Imperium, Breacher Squads were phased out, though it was part to the strains of extreme fights in the battlefields the Breachers operated that resulted in constant maintenance issues. This also what propelled the cog boys to accelerate the development of Terminator Armour, which, although more effective in close-quarters battle, was both much more time-consuming and resource-intensive to manufacture and train marines in its operation than Breacher Squad equipment.
Although the organisational unit of the Breacher Squad no longer exists in the 41st millennium; the combat bonuses that such shields grant against melee-heavy forces such as the Orks and Tyranids are substantial, even more so in confined environs. Therefore depending on the battlefield requirements, be it Zones Mortalis or acting in a Siege Vanguard, Tactical Squads can still be issued with Boarding Shields or "Siege Mantlets" and can be equipped with void hardened armour where necessary and largely fulfill the same role as the old Legion unit. This is probably the reason why "Breacher Squads" no longer exist as a permanent unit, as codex Tactical Squads are experienced warriors in multiple fields who can just fulfil the role whenever required of them, which is more efficient in a Chapter of multi-role companies.
Specialist Breaching Formations
Phalanx Warders
In the Imperial Fists Legion, breaching and siege units formed a core part of their organisation and training. The VII Legion may have even pioneered the use of Breacher Squads as a regular formation, as Horus was said to have noted their effectiveness in battle during the early Great Crusade and adopted the practice of arming some of his squads with breacher shields afterwards.
The Phalanx Warders were an elite company of Breacher-trained Astartes who were nominally assigned to the defense of the Phalanx, though they were frequently dispersed to other VII Legion units across the galaxy to keep their skills sharp and to also provide an immovable line of defense. They were most often found in the kind of close quarters ship-to-ship boarding actions which the Imperial Fists excelled at. Indeed, the unique Imperial Fist Rites of War frequently feature Phalanx Warders as a core part of their battleforce. Aside from their shields, the Phalanx Warders carried power axes and bolters as standard kit, though they also employed thunder hammers, special weapons such as meltas and flamers, and Solarite power gauntlets.
Medusan Immortals
A specialized unit of Breachers that were unique to the Iron Hands Legion, Immortals were Iron Hands that were shamed by experiencing defeat or suffering grievous wounds (but not grievous enough to put them in a Dreadnought) and who were given Breacher equipment along with very extensive cybernetic augmentation. Pre-Heresy Iron Hands had a very rigid and extreme set of standards that promoted strong-willed individuals and weeded out weaklings. If a marine was, as mentioned before, shamed in either of the above ways, the Primarch would strip them of any ranks and titles, give them a serial number, and remove them from their original companies to fight on their own as, essentially, living marks of shame.
For such marines, being an Immortal was the only way to gain redemption in the eyes of their Primarch and brethren, usually in the form of an honourable death in combat and thereby a final chance at glory. Formations of Immortals were often treated as "Forlorn Hope" (i.e., units that were first into a breached fortification or the entrance into a Zone Mortalis area and not expected to survive due to the often-ferocious defenses and cramped environments they would encounter), and accordingly the Immortals fought with determination in warzones so extreme that even standard Breachers would have had little chances at coming back alive.
During the Horus Heresy, the ranks of Medusan Immortals swelled dramatically with bitterhearted volunteers after the Dropsite Massacre, as the severely-depleted Iron Hands survivors there saw the deaths of Ferrus Manus and the Clan Council as their greatest shame and humiliation, a failure beyond any repentance or atonement to expunge. However, those who came from Clan Morragul were allowed by Autek Mor to fight alongside their brothers (which was actually unusual). And unlike other Immortals, they were allowed to wear a red talon symbol to further distinguish themselves (and these marines were the base on which the Red Talons Chapter would be founded).
Despite these inauspicious beginnings, Immortals were not in fact generally equipped as expendable units, useful only to shed as much blood of the enemy's as they could before drowning in their own while carrying only the weapons they could scavenge, with more capable equipment assigned to "worthier" units. The fact that Immortals had extensive cybernetic augmentations allowing them to infrequently endure wounds that would have downed unaugmented marines while remaining combat-capable, as well as the fact that they could all be equipped with the increasingly-uncommon Volkite Chargers, allowed them to retain a certain tactical niche within the environments they were suited for, and allowed Iron Hands forces another option when Terminator armour was too valuable (as it inevitably became after the Dropsite Massacre) to deploy in close-quarters battle situations. It is also evident that enough Immortals survived repeated engagements to begin to adapt to and specialize in their preferred combat environments, eventually learning to forego the usual "run them down" tactics ill-suited for their Void-Hardened Armour, and instead adopting "gun them down" tactics against fleeing enemies, so whatever Ferrus Manus originally intended, the record is clear that the Immortals eventually became a useful and experienced combat asset in their own right.
Invictus Suzerains
As a precursor to what would become the Honour Guard of the 41st millennium, the Invictus Suzerains were the elite troops of the Ultramarines legion who were chosen from the ranks of other squads who have been singled out for potential promotion to command roles. It was their duty to act as the embodiment of their Primarch's will and served either him or his Tetrarchs directly, therefore they acted as a veteran cadre that could be attached to units in any battlezone.
Their wargear was symbolic, but undoubtedly powerful. They were gifted with Artificer Armour and Boarding Shields to represent them holding disorder at bay, and came with a Legatine Axe with which to enact swift justice, therefore they also acted as arbitrators and civilian peacekeepers.
