Hardskins

You expect me to believe that you just lost a full hardskin? That shouldn't even be possible! It's the size of my parents house in the northlands, where the ____ did it go, hm?
Are you implying that it got up and walked away?

-Overheard on a Docran military base

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Hardskins are a middle ground between common infantry and the heartier walkers seen on the modern battlefield. Bipedal war machines, usually operated by one or two pilots, that are prized for their heartiness and flexibility within combat situations.
When they first made their appearance on the battlefields of the Great War, newspapers called them the "clockwork soldiers", though it is an erroneous title, being machines fueled by Oil, the theming stuck, leaving Hardskin pilots jokingly named as 'watchwalkers'.
While not as heavily armored as modern tanks and other combat walkers, Hardskins make up for it with their humanoid shapes and the fact that they're a weapon that walks upright. They can operate in any environment, from mountains to urban, and can do things like crouch and lie down to reduce their profile. Their armament is also something that is incredibly flexible, considering a mechanic crew doesn't really have to do anything to change out a hardskin weapon. The Hardskin simply has to put down what it's holding and pick up the new equipment.

Discovery

While the Great War raged on, the nations of the world spent money and time in an attempt to develop new weapons of war to slay their enemies, and protection to act as a bulwark against foes.The Hardskin was an invention in the later category, though it is by no means defenseless, given the wide range of arms that one can wield.
They were first fielded by Qilonn, though their desings would not become the most famous, nor the most iconic. They were, however, immediately effective against the infantry positions that they were deployed against. As effective as they were, modern anti-armor capabilities have diminished their head-on abilities, though their flexibility and operational abilities have kept them present on the battlefield.