OK, here is fluff for why the Forth use bipedal machines.
In the dim past of their history, an alien spaceship crashed on the Forth home world. As this was obviously from a very advanced culture, the ancestors of the Forth tried to understand the message that it was sending them.
They sorted through the contents of the spaceship, and found the creature's bodies puzzlingly different in that their biology was bipedal. They noticed that there were larger devices that were made to fit as extensions of the creatures legs (found in a cargo area) that extended and enhanced the mobility of the creatures. A cult soon grew around these strange alien beings, none of whom survived the landing.
Knowing that the spacecraft was from a superior Race, the early Forth tried to emulate this efficient biological design into their later transportation machines once they went through their industrial age. When the level of technology of the Forth allowed the production of advanced war machines, they too were patterned off of the ancient bipedal design. The first views of an Arachnid Warrior gave them the added idea of a four-legged machine for added stability.
What the Races that later met the Forth did not know and would never find out was that what gave the Forth their mobility and their inspiration to reach out to the universe was the equivalent of a circus transport with stilt-walkers aboard. This alone set the Forth on a path to gain technological ascendance to threaten the other sentient inhabitants of space.
The universe is not without a sense of humor and irony, it seems. :wink: