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 Praevian
Consul Praevian were a type of Astartes commander during the Great Crusade and Horus Heresy and a variant of a 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.