This mecha paper model is Big Fau, first seen in Roger's memories of "The Event" in "R.D." and made its first full appearance in "The Third Big" of the animated television series The Big O, the papercraft is created by WebDude.
It was called "The Third Big" and the "White Big" by Vera Ronstadt, and Alex Rosewater believed it to be the most powerful of the "Big" Megadeus.
The parts for Big Fau were transported within the three Foreign Megadeus that fought Big O offshore of Paradigm City in "R.D." and "Roger the Wanderer". After they were defeated, their remains were recovered by the Paradigm City science division and the relevant parts re-assembled. The Megadeus, however, lacked a memory core. Ultimately, this vital component was disregarded, and Big Fau was activated and piloted by Rosewater.
During the first battle against the Union Megadeus Bonaparte, Alex controlled the Big, allowing him to destroy Bonaparte with ease. Once Bonaparte was vanquished, however, Big Fau's dorsal beams disobeyed Alex's orders and began to attack the domes. In response to Alex's claim that he was Big Fau's dominus, Big Fau displayed the message "Cast in the Name of God... Ye Not" before shutting down. Later, Big Fau was jury-rigged into working by Jason Beck using R. Dorothy's memory core, finally completing Fau and allowing it to declare Rosewater "Not Guilty."
Fau is also the only known Megadeus to fuse with its pilot, Rosewater, via a series of cables plugged into its pilot's back - although Big O attempted to do the same with Roger, and Big Duo killed Alan Gabriel with the message "Ye Guilty". Big Fau was eventually disabled by Big O's Final Stage at the end of the "The Show Must Go On" and removed by Big Venus.
Judging by Big Fau's streamlined appearance, its turbine wrists, its ability to travel effortlessly through the water, its conning tower-shaped head as well as a conning tower on its back, and the flexible periscope-like laser cannons on its back, it is likely that Big Fau units were used primarily for naval combat, functioning as heavily armed amphibious craft, and providing water support for the Big O land units and Big Duo air units.