Horizon Zero Dawn Explained: What Does The Ending Really Mean?

5. What Are The Machines? Why Are They Hostile?

Being GAIA and the APOLLO systems are embedded with the knowledge of Earth circa 2060, that includes biological designs on every living thing, plus all applications of said living things to the world at that time, and how they would then factor into the 'perfect world' GAIA is striving to create.

Basically, if you were designing the world from the ground up right now, you'd detoxify the seas and perhaps restore certain extinct species', but keep the base likes of trees, mammals, plants etc.

To that end, once all organic matter has been wiped out by what became known as the 'Faro Plague' of machines, repopulating a planet that has withstood such drastic terraforming requires a different set of creatures. As such, the robo-animals and 'dinosaurs' we see wandering around the world are the best approximation - from an A.I.'s perspective - as to which animals are the 'cream of the crop' so to speak, for whatever biosphere is active at that time.

To that end, the very fact they have different components, oils, fuels and elements within specific containers on their bodies, is down to their 'role' in transporting them across across the land in that cycle, aiding in this recreation of a survivable ecosystem, as GAIA dictates.

Whilst they were forever supposed to respect human life and work alongside it, when GAIA is forced to shut herself down to prevent an attack from HADES, it results in the recent 'derangement' various NPCs mention, where the breakdown of the creatures' core functions have started treating everything as hostile.