Mechs are war-built robots and the antagonists of Enslaved. Originally, the Mechs were constructed for a war to fight an opposing forces, but after it ended they continued to follow their programing, continuing to hunt down and kill any Humans they could find. There are several different types of Mechs, each having a unique way of attacking its victims.
It is unknown what started the war that destroyed human civilization, or any other details, but what is known is that this war saw a very wide variety of mechs with individual programming, so even after the war the mechs stuck to their original programming, kill any and all living humans.
In spite of their feral nature some mechs can still be reporgramed and used by humans, as seen in the case of the mechs on Pyramid's slave ship and the dragonfly used by Trip.