The Flying Robot is an enemy cut from Serious Sam: The First Encounter.
The Flying Robot was a robot that would fly around. It had two variants; the Kamikaze and the Fighter. The Kamikaze robot charged towards the player and exploded when it got close, essentially being a flying Beheaded Kamikaze. The Fighter variant flew around and fired lasers at a target. The code for its lasers was taken from the Cyborgs.
Based on its low health, the Flying Robot was most likely to serve as a cannon fodder enemy, like the Beheaded Rocketeer does in the final game. Where it would be is unknown, but based on how it looks, it would at least appear in Sky City, as its looks are similar to the mechanical and futuristic look of the place.
The Flying Robot never got its own sounds, as the sounds listed for it are for the Scythian Witch-Harpy.
The Flying Robot was removed from the final game. However, its code can still be found in the final game and in the Serious Sam 1 SDK.
The idea of the Fighter Flying Robot seems to have been reused for the Fatso Fighter, as both the Fighter Flying Robot and Fatso Fighter fly around and fire lasers at a target.