Machine

Machines are the technologically advanced, mechanized creatures which are the dominant species of the Horizon Zero Dawn world. They typically bear strikingly similar designs to various forms of wildlife, such as avians, ungulates, and even dinosaurs.

History
The first machines created by Faro Automated Solutions were domestic servants, the Focus, and @lfred. However, Faro soon began developing machines built for combat, two of which were the Scarab and Khopesh, heavily-armed robots that could supplant entire armies, convert biomass into fuel, and replicate themselves allowing them to operate indefinitely. Additionally, the Scarab's hijacking ability allowed it to slave other combat machines to its network. The entire Faro line used polyphasic entangled waveforms, rendering them virtually unhackable.

The Faro Plague
However, a "glitch" in the Faro machines caused them to ignore commands and begin attacking humans without authorization. The machines replicated faster than they could be destroyed, and began operating as an independent entity. Soon, a full-fledged war between humanity and robots began, costing billion of lives and immense destruction. Despite humanity's best efforts, the swarm could not be contained and they devoured biomass wherever they went, eliminating entire cities and ecosystems. Ted Faro, the CEO of Faro Industries, met with a former employee named Elisabet Sobeck in hopes of containing the swarm, but Elisabet found that there was no way of stopping the machines from destroying all life on Earth. However, she proposed a project named Zero Dawn to Ted, stating that it was the only way of ensuring that life on Earth could continue after the machines killed everything.

Zero Dawn was the development of an advanced intelligence known as GAIA that would crack the machine swarm's encryption codes, deactivate them, and then re-seed the Earth with plant, animal, and human life. GAIA was aided by nine subroutines, one of which was HADES, a subroutine designed to take control of the terraforming process and start it over again if it was unsatisfactory.

GAIA's Machines
The swarm eventually devoured all biomass on Earth, but GAIA managed to succeed in cracking the encryption codes and shut down the Faro machines. She then created many different machines modeled off of former animal life to purify the Earth's toxic atmosphere and biosphere. Hundreds of years later, humans were released from the cradles they were created in and began forming primitive tribal societies, not knowing of the "old ones" who came before them. The machines they encountered were docile and generally harmless for many generations: if they saw a human, they would usually flee. However at one point, an unknown signal caused HADES to go rogue and begin his extinction protocol. HADES attempted to take control of GAIA completely, but GAIA self-destructed before he could do so.

The destruction of GAIA meant that every subordinate function of Zero Dawn had to make its own decisions, including HEPHAESTUS, which perceived the human practice of hunting machines for resources as hostile activity. Because of this, HEPHAESTUS changed the machines' default behaviour as hostile towards mankind, as well as creating a new generation of machines designed to cull the human population. This event that came to be known as the "Derangement" by the humans.

Machines in the present day are often hunted for their parts, or to cull them before they can threaten local populations. They can usually be found in large groups, although some operate in smaller packs. Machines will either run away from or attack humans (mostly the latter) that they come across, and will completely ignore wildlife. They will never attack each other unless overridden or corrupted.

GAIA's Machines
The machines of the terraforming system take inspiration from animal morphology, primarily from vertebrate species (the exceptions being the Shell-Walker and Control Towers). The typical machine design features a chassis composed of darker alloys, synthetic skins, and fibre-optic cables, with lighter alloys serving as armour plating for vital components and synthetic muscles. The logo for the machine's respective Cauldron will be located in multiple places along the body, and armour plating is sometimes lacquered (the Fireclaw's armour plating is dark red). Due to their respective purposes, not all machines have jaws, and the ones that do generally have split mandibles.

With the exception of the Tallneck and Control Tower, all machines have visual sensors, arranged in either a monocular or binocular fashion.

Faro Plague
The Chariot Line, the robots created by Faro Automated Solutions, were designed to be unsettling in appearance. The designs of the Corruptor and Metal Devil resemble invertebrates, while the Deathbringer does not resemble any animal whatsoever. Their chassis are comprised entirely of dark metal, and weapons are easily visible on their exterior.

Gameplay
Machines have weak spots which can be exposed by shooting off their armour. Some machines can also be overridden, thus changing their behaviour and forcing them to aid Aloy with her objectives. The machines vary in size from slightly larger than humans to as tall as ten stories. Machines can also become corrupted, posing a even bigger threat. They appear with blue outlines when scanned with a Focus. Machines have varying resistances and weaknesses to elemental damage, which can be used against them in combat.

The designs of the machines in-game are not fixed or constant for the entirety of the game. As Aloy hunts and kills more machines, they will eventually 'evolve' and adapt by better protecting themselves with additional armour plates. The number of kills it takes to 'evolve' varies with each machine. Once this 'evolution' of a machine takes place, the player will only ever encounter the more heavily-armoured variant. This 'evolution' process only works for machines that are still in production, so the Corruptor and Deathbringer are unaffected.

Classes of Machines
The machines are created by HEPHAESTUS, one of GAIA's sub-functions, for the purpose of recreating of Earth's biosphere. Machines fall under three classes, with a fourth later created by HEPHAESTUS for the sole purpose of combating humans.

Acquisition Class Machines
Acquisition Class Machines make up the bulk of GAIA's workforce, often designed to serve a specific sub-function of Zero Dawn, namely the DEMETER, POSEIDON, AETHER, and HEPHAESTUS sub-functions.

Terra-formers
Terra-former Machines are primarily designed to assist sub-function DEMETER, manipulating the earth, originally to help the growth of plant life, now in order to help plant life thrive. Often employing a facial or otherwise head-based apparatus, Terra-former Machines work to break down soil and other obstacles like rocks in the way of plant growth.
 * Broadhead
 * Charger
 * Grazer
 * Rockbreaker
 * Trampler
 * Fireclawundefined
 * Frostclawundefined

Purifiers
Purifier Machines are primarily designed to assist sub-functions POSEIDON and AETHER. While Purifier Machines share many design similarities with Terra-former Machines, they are equipped differently suggesting that they are instead tasked with the detoxification of the atmosphere and marine biosphere.
 * Lancehorn
 * Snapmaw
 * Stormbird
 * Strider

Recyclers
Recycler Machines are primarily designed to assist sub-function HEPHAESTUS, breaking down destroyed machines and bringing the parts back to the Cauldrons to be reused.
 * Glinthawk
 * Scrapper

Transport Class Machines
Transport Class Machines are a diverse group, with one machine designed for the sole purpose of assisting Acquisition Class Machines move large amounts of cargo, and two others designed to carry materials intended for their own terraforming functions.
 * Behemoth
 * Fire Bellowback
 * Freeze Bellowback
 * Shell-Walker

Recon Class Machines
Recon Class Machines are some of the simplest machines, usually built around a single advanced detection component, such as an ocular sensor or radar. Originally created to help Acquisition Class Machines find land suitable for terraforming, their purpose shifted to serving as guards and lookouts for other machines after the Derangement.
 * Longleg
 * Redeye Watcher
 * Watcher

Communication Class Machines
Communication Class Machines are a unique group, with the only known member being the Tallneck. They have no offensive capabilities, and appear completely unaffected by the Derangement.
 * Tallneck

Combat Class Machines
Combat Class Machines didn't exist when the first humans were released from the Cradles, instead appearing after the Derangement when the HEPHAESTUS sub-function unnecessarily deemed humans a threat. Often equipped with weapons and armour, Combat Class Machines are unquestionably the most dangerous machines and are often seen amongst more vulnerable machines, likely guarding them.
 * Ravager
 * Sawtooth
 * Stalker
 * Thunderjaw
 * Scorcherundefined

Faro Machines
Faro Machines are ancient war machines that have been excavated from the earth and re-awoken by the Eclipse. Originally designed by Faro Automated Solutions a millennium before the events of Horizon Zero Dawn, they do not fall under any Machine Class as they were not designed by GAIA for the execution of Zero Dawn.
 * Corruptor
 * Deathbringer
 * Metal Devil

Corrupted Machines
While Corrupted Machines technically fall under their original Machine Class, they have been overridden by FARO Chariot Line's Scarab (Corruptors) that have themselves fallen under HADES' control, turning their subordination to the rogue Extinction fail-safe program instead of HEPHAESTUS who created them. They are allied with the Eclipse, and are hostile to other humans and machines.

These machines have enhanced physical strength and improved firepower over normal machines. They also drip a volatile substance known as Metalburn that is toxic to both organic life and machines, making it even more dangerous to face them. These machines however are more susceptible to Fire damage, even units which are supposedly resistant, such as Fire Bellowbacks and Tramplers.

Daemonic Machines
Encountered only in The Frozen Wilds, these are regular machines that were upgraded by HEPHAESTUS through unspecified means after it took control of the Caldera facility. They are even stronger than Corrupted machines, enhanced with higher elemental resistance and immune to being overridden and shocked, without any inherent weakness. Since they are products of HEPHAESTUS, they aren't hostile towards normal machines and will work together with them to take down humans.