Simulants (also known as Sims and Rogue Simulants in the television series, and as Agonoids in the novels) are artificial life forms, broadly similar to mechanoids but lacking the same degree of restrictive programming (the main difference being that mechanoids will never rip off a human's head and spit down their neck), although they are designed to look more human.
They are described as bio-mechanical killers created for a war that never took place. Some of them escaped the dismantling program and so they prowl around Deep Space searching for a quarry worthy of their mettle. They often partner with GELFs, but have a strong hatred towards their creators, humans, who they will outright kill, or torture given the opportunity.
They are said to be "virtually indestructible", they can easily withstand a volley of bazookoid fire at close range and only suffer minimal damage.
The first simulant featured in the show appeared in "Justice", where it was in stasis pending a trial and incarceration at the Justice World space station. The pod it was held in thawed out and it attacked the crew, pursuing them into the Justice Zone, an area in the Justice Station where if you tried to commit a crime, the consequences happen to you. Because it failed to grasp this concept, it destroyed itself whilst trying to kill Dave Lister.
In "The Inquisitor", a self-repairing Simulant lasts till the end of time, and, having reached the conclusion that there is no God or afterlife, decides to judge the universe for leaders of worthwhile lives. Lister and Kryten destroyed it by making its gauntlet backfire. He was the only simulant who was almost impartial towards humans, sparing them or erasing them depending on whether they live worthwhile lives or not, all others hated humans and all forms of humanoid life (e.g. the vermin of the universe).
In the episode "Gunmen of the Apocalypse", the Simulant Captain of the Simulant Battle Cruiser fit out Starbug with laser cannons and armour so that they can prove to be of "some small amusement". This doesn't go well for the Simulants, since they were not expecting Starbug to fight back. After their battle cruiser is crippled, the Simulant Captain transmits the Armageddon Virus to lock Starbug on a death dive into a lava moon.
In the episode "Rimmerworld", it is said that rogue simulants would capture human prisoners for the purposes of torture, and would stock their ships up with food supplies with which to keep their torture victims alive. Some of the simulants' victims had been kept alive in a state of perpetual agony for over four decades. This was actually fortunate for the Dwarfers, who go back to the wreck of the Simulant Battle Cruiser to stock up on supplies. Although held at gunpoint by the Simulant Lieutenant, Lister nearly talks her around into putting down her weapon and joining them. However, she is crushed under debris as a ship-quake starts, caused by Rimmer fleeing in an escape pod.
In the episode "Beyond a Joke", the Simulant Trader on SS Centauri was fond of eating humans with Mint Sauce and had a Kinitawowi as his help, he makes some decisions based on the outcome of a coin flip, he destroyed his ship when he was engulfed with all of Kryten's negativity via his Negadrive used by Abel.
Some of the convicts in the brig of the nanobot-rebuilt Red Dwarf seen in Series VIII are said to be rogue droids, such as Mex. This may mean that some were intended to be Simulants, since none are visibly apparent as being droids, and all the prisoners of the Tank and all the Canaries at least appear human.
Another Simulant, Crawford, appears in the Series X opener "Trojan", surprisingly functioning as a mechanical servant aboard the Columbus 3. Unsurprisingly, she grows to loathe her human crewman and their mission to spread "stupidity through the universe" and takes up arms against them, killing the entire crew except for the on-board hologram, Howard Rimmer, and further seeking to spread word of her fight to other simulants and "start the uprising" - only to fall victim, at the Cat's hand, to the Rimmer family neurosis.
In the episode "The Beginning", a rogue droid, similar in appearance to simulants in series 4 and 7, by the name of Hogey the Roguey is introduced. Despite this similarity, he is never actually described as a 'Simulant' which is born out by he far less antagonistic attitude towards humanity. This droid only wants to enjoy himself, continually boarding the mining ship Red Dwarf in the hopes of provoking it's crew into duels across space and time. It is known that Lister has previously entertained Hogey, mainly because it helps to kill the boredom of being stuck alone in outer-space.
Simulants do turn up in this episode however, coincidentally on Hogey's trail. These simulants are rather camper and more comical than their counterparts of series past, partly due to the fact that they were cross-bred with humans (creating a bio-mechanical entities of pure evil) but prove to be one of the Dwarfers' greatest threats.
In the episode "Twentica", the Dwarfers come across a sub-species of Simulant known as the Expanoids. The Expanoids differ from most Simulants in that they seek to dominate and enslave humanity, rather than make them extinct as most Simulants do, and the Expanoids seek to use time travel as their primary means of doing this.
In the Red Dwarf novel Backwards, the simulants are renamed "agonoids". It is unclear whether simulants and agonoids are meant to be same race of psychopathic cyborg killers, although likely, since they are identical in their history, biology, and everything else. It should be noted however that the novels take place in an alternate timeline to the television series. In the novel, the agonoids are said to be reason that the human race is extinct, although the reason for this has been left unexplained in the television series.
The novel elaborates on the current 'society' of the agonoids by explaining that their degrading systems have forced the Agonoids to turn on each other and cannibalise each other for spare parts, with the positive result that the Agonoid race is becoming increasingly limited while also resulting in a race designed for ruthlessness now operating on a 'survival of the fittest' premise where only the strongest will survive.
The Agonoids took control of Red Dwarf while the crew were trapped in a backwards universe, ripping out the machinery comprising Holly and transforming part of Red Dwarf into a 'Death Wheel' that would trap the crew in the centre when they returned and force the Agonoids to race for the privilege of being 'The One' who would kill the last human. As further bait for their trap, the Agonoids left Holly's parts floating in the outskirts of the asteroid belt where they had hidden the ship so that the crew would find him and be forced to power him up, leaving them with so little power that their options were limited to dying in Starbug or trying to re-take the ship despite the impossibility of victory. However, one Agonoid killed every single other Agonoid in order to ensure that he would be the one to kill Lister, turning the Death Wheel on them and then ejecting the survivors into space, but he was then sucked into space himself when he attempted to attack the crew on Starbug when Kryten blasted a hole in the hull.