Sentinels

The Sentinels are specialized mutant-hunting machines created by Dr. Bolivar Trask as a solution to what he felt was a growing problem caused by superhuman mutants; however, the Sentinels soon grew out of hand and planned on taking over humanity- it was only due to the intervention of the mutant superhero team known as the X-Men did the Sentinels plan for domination topple.

However, the damage had been done, and many other villains and misguided souls created new versions of the Sentinels- some driven by hatred of mutants, others simply trying to make the world safer -these Sentinels varied largely in size, function and danger but all of them shared a common trait: they were merciless hunters of the mutant race and a symbol of fear and hate in the eyes of many.

In addition to the legions of lesser drones, there have been a number of Sentinels who have evolved beyond their normal programming and have become super-villains in their own right. Examples of these elite Sentinel units would be Master Mold (the supreme leader of the Sentinels in the early years), Nimrod (a future Sentinel of immense power) and Bastion (the mystical-fusion of Master Mold and Nimrod).

Prime Sentinels
The Prime Sentinels are a variant Sentinel class created by Bastion and are separated from all other Sentinel classes by being cyborgs.

The Prime Sentinels were designed to hunt down and eliminate mutants just like their robotic counterparts, but the fact that they were part organic (with the ability to feel, yet the inability to fight against their control) made them a much more problematic threat to the X-Men, as they could not simply kill them without serious ethical debates. A way was found to put a stop to Bastion's creation of Prime Sentinels, though whether or not they'll show up again is unknown.

Wild Sentinels
Arguably one of the most infamous Sentinel classes of all time and one of the rarest- the Wild Sentinels gained international infamy when they were actived by Cassandra Nova.

The omnicidal madwoman proceeded to use the Wild Sentinels to massacre the nation of Genosha- killing millions of mutants in one of the greatest massacres the X-Men ever faced (at least during that time period).

The nightmare of the Wild Sentinels would resurface once more when Danger unleashed one on the X-Men again- bringing back more than a few bad memories.

The Wild Sentinels do not seem to be as easily mass produced and, given the terrible loss of life these particular Sentinels caused, it is unlikely that they would be utilized by most factions (of course, this doesn't exclude them being used by psychotic terrorist groups, such as The Right or Purifiers).

In media

 * In the X-Men animated series from the 1990's, the Sentinels are the main antagonists of the first season. The robot's designs are generally the exact same as the comic versions. They are created by Henry Peter Gyrich and Bolivar Trask to hunt mutants. After some Sentinels failed to stop the X-Men from escaping a government facility (two Sentinels critically injured Morph, but he survived), the President of the United States told Gyrich and Trask to cease the Sentinel program, as it was extremely controversial to keep them built up. The X-Men find the other Sentinels in a factory and destroy them. Gyrich and Trask build Sentinels for an island called Genosha, where they serve as guards to stop mutant prisoners from leaving. Mastermold was also built during this, as he would manufacture the Sentinels in his own body. X-Man Storm causes a flood that destroys the dam the mutants were building, also destroying Mastermold. In the season finale, Mastermold is re-built, but he turns on Gyrich and Trask due to knowledge that humans were capable of producing mutant offspring. He sent Sentinels to kidnap world leaders and put robot minds in their heads so he would control the world, but he first had to take on the X-Men. He rises from the mine in which he lay, where Professor Xavier drives the Blackbird jet (filled with explosive chemicals and devices) into Mastermold, destroying the robot. In Season 3, Mastermold's head survived the explosion and he gathered several Sentinels to steal metals with which he would build his new body. He also desired to remove Xavier's brain and use his mind-powers for himself. The X-Men (accompanied by a traumatized Morph) go to the cave where the Sentinels are hiding to save Xavier. Morph uses a stalactite to impale Mastermold's head, causing an explosion that buries the Sentinels in rubble.
 * The show X-Men: Evolution features Bolivar Trask as a former S.H.I.E.L.D. scientist who managed to build a giant Sentinel to assault mutants. This Sentinel looks much more menacing than the comic-book version, having more weapons to use at its disposal. It is featured in the Season 2 finale, where, to Trask's dismay, Magneto releases the Sentinel on New York City, forcing the Brotherhood of Mutants and the X-Men to fight the Sentinel while a news helicopter watches, thus revealing mutants to the entire world. Magneto reflects missiles back at the mechanical monstrosity, but a revenge-bent Scarlet Witch freezes Magneto in place, making the damaged Sentinel fall on top of Magneto, where it explodes, destroying the Sentinel and presumably killing Magneto. In Season 3, Toad and Scarlet Witch discover that Quicksilver saved Magneto at the last second. In Season 4, S.H.I.E.L.D. releases Trask from prison and get him to build more Sentinels to battle Apocalypse. While they seem effective at first, the Horsemen of Apocalypse (Magneto, Professor X, Mystique and Storm) make short work of them. The X-Men and the Brotherhood take on the Horsemen and Apocalypse, stopping his plan to turn all humans (that survived the following event) into mutants. The series finale hinted at a future version of Nimrod leading an army of Sentinels to dominate the world.
 * In a manner that homages the Terminator films, the Sentinels (which still look like their comic-book counterparts) appear in the TV show Wolverine and the X-Men. This show features the Days of Future Past timeline where Sentinels rule the world and they hunt all mutants. After Professor X enters a coma and wakes up 20 years after the explosion at the X-Mansion, he sees a future where (presumably) all humans are destroyed and all mutants are hunted by Mastermold and the Sentinels. Meanwhile in the present-day, Senator Kelly has a Sentinel robot built called the "Sentinel Prowler" (which resembles the Spider-Slayers from various Spider-Man media) that would later be mass-produced. The comic-book Sentinels are being built in the present as shown in later episodes. In the future, it is revealed that something happened which caused fiery flames to consume the world, which led to the Sentinels rising up and blaming mutants for causing this. It is revealed towards the end of the series that this was caused by Dark Phoenix's host body (Jean Grey) to unleash her full powers, unintentionally destroying the world in the process. She also caused the explosion at the X-Mansion. Wolverine eventually shows Magneto and Kelly to the comatose Professor X who uses his powers to show the two what their war will end with in the future. Kelly stops Sentinel production, but Magneto wanted to do something else. He got Mystique to disguise herself as Kelly and re-build the Sentinels, which would attack Magneto's kingdom Genosha. He would then control the Sentinels and get the mutants of Genosha to despise humanity. The plan seems to work without flaw until the Phoenix comes along and injures Magneto, destroying the Sentinels. In the future, Mastermold (as a feminine-shaped robot) leads the future X-men to a trap where she would execute them all. Right before that happens, however, the redeemed Emma Frost absorbs the Phoenix into herself, turns into diamond and explodes into diamond chunks, releasing the demon into the atmosphere and preventing the Sentinel future from occuring.


 * In X-Men: The Last Stand, Sentinels are simulated in the simulator room at the beginning of the film.
 * In the film X-Men: Days of Future Past, the Sentinels are the quaternary antagonists. In a similar manner to Skynet from the Terminator movies, the Sentinels were built by Bolivar Trask to track and neutralize mutants. However, they rose up over humanity and conquered the world, placing mutants either in hell or in prison camps, the same thing they did in the Wolverine and the X-Men show. Kitty Pryde sends Wolverine back in time from roughly 2040-2050 all the way back to 1973, when Mystique had killed Trask and was captured by the government for experimentation. Wolverine got Magneto, Beast and Professor X to help him stop Mystique, which they manage to do. Towards the end of the film, Magneto inserts metal bars into the Sentinels of the past and gains the ability to control them. At a press conference where Trask unveils the Sentinels before the White House and President Nixon, Magneto gets the Sentinels to accompany him as he drops a football stadium around the press conference area, with one Sentinel inside and the others on "lookout". Magneto makes his speech to the world about people fearing mutants due to their superiority while, at the same time, the Sentinels of the future are going over to the abandoned temple where the time-travel thing was happening. The Sentinels all but kill all the X-Men there, while Mystique prepares to shoot Trask again. Professor X talks her out of it. Right as she drops the gun, everyone at the temple in the future vanishes into thin air, stopping the future. Trask is arrested for selling military secrets and the Sentinels are destroyed.

Trivia

 * In the film X-Men: Days of Future Past, Bolivar Trask said he named the Sentinels after the guardians of the citadel. In actuality, it was taken that he meant The Citadel, the real-life military academy in SC, and that its guards were dubbed "Sentinels".
 * The Sentinels are eerily similar to Skynet from the Terminator franchise; both are sentient AI that were tasked with military protection over the world, but judged humanity as cursed and created a future where humanity is about to be destroyed. Skynet built the Terminator robots, much like how Master Mold built the Sentinels, to carry out the deed.