Consul
Legiones Consularis, or Consuls, were a rank bestowed to Space Marines during the Great Crusade and Horus Heresy. They were not Legion Consul. They were identified by their specialization in many esoteric arts of combat and war. Called upon to use their mastered arts when their Legion needed them, with the outbreak of the Horus Heresy these officers would face their grandest trial. As one can imagine, there is a metric shit ton of different variants of Consuls. Unfortunately, the only other Consul that we have sufficient info for and has a model, is the Consul Praevians.
Variants
Consul Champion
The predecessor to the Company and Chapter Champions of the Chapters, their role has seen the least amount of change. After all, the Champion was always meant to be an exemplary swordsman among their brothers and fight the greatest foes. How else were they to improve that?
Consul Chaplain
While the Imperial Cult was not established yet, there were still people who needed to shout really loudly to inspire zealous anger. Similar to their successors in black, these chaplains would inspire hatred in their charges and wield a master-crafted Power Weapon - though unlike the Crozius Arcanum synonymous with the later iterations, these could be any sort of power weapons than merely mauls.
Consul Librarian
Due to the Council of Nikea, the Librarius was dissolved among the legions, but as many can attest, this was not for long.
In most cases, there may have been cases where a Psyker was elevated to a more senior position to make use of their unnatural powers, but as time passed, it became clear that the powers of psykers were one of the only ways to defeat the foul warpspawn that the traitors summoned...or to summon them. Following the Heresy War, Guilliman would re-establish the Librarius as a structure in the Chapters.
Consul Esoterist
Among the psykers of the legion, only a growing few of them dared to dive deeper into the unknown fields of Warpcraft, but it proved to be incredibly effective, if in varying ways.
While the loyalist legions had Esoterists who studied how to banish daemons outright, the traitor legions discovered ways with which they could summon the forces of Neverborn into reality with their own power.
Consul Praevian
The Consul Praevian was a type of Astartes commander during the Great Crusade and Horus Heresy and a variant of the Consul.
Trained on Mars by the Mechanicum these commanders were the keeper of the Legion's robots, including Battle-Automata given honorary induction into the Legion's ranks. On the field of battle they marched at the forefront of Battle-Automata forces, guiding them through the engagement.
They were often chosen from Veterans whose injuries required extensive Bionics. Known to be solitary individuals, they rarely advanced up the chain of command. Some Legions used the rank as a dumping ground for warriors seen as unfit for other duties. However, in the Iron Hands (No surprise there) and Salamanders, the rank was seen as a great honour. On the traitor side, Iron Warriors also make great use of them due to their command over siege robots. As they are heavily augmented, they wore what is essentially Artificer Armour for defence, but by the time of Horus' clusterfuck benanza some of them had to wear Mark V: Heresy Armour.
Some has considered viewing them as a predecessor of the Techmarine, but with each of them in command of a mini robot-army.
On the Horus Heresy tabletop, they are the Master of the Legion's Automata. He is a Support Officer, so cannot be used as a compulsory HQ and thus cannot be in an allied contingent. Comes equipped with a Cortex Controller, Cortex Designator and can't take anything that makes him bulky. First, you choose a type of Battle Automata (only Castellax and Vorax at this time) to accompany him, forming one unit with MCs (so he can't go to ground) that he can't leave. Any time the Praevian hits an enemy unit with a ranged attack, he grants Preferred Enemy to the Automata in his unit for that turn (so give him a bolter). Upgrade them with Enhanced Targeting Arrays for 2+ re-rollable accuracy for your weapons.
Legion Primus Nullificator
As the Heresy ground on, the Loyalists tried a great many methods to ward off the daemons. One of these would be the Nullificators, a small force of marines who adorned their power armour with Hexagrammatic Wards and used superstition and normally unsanctioned weapons like disintegrators. Leading these forces would be the Primus Nullificators, psykers who would be specially trained to purge daemons.