Overlord Momus (InstanceTopic, 8)
From Compile Worlds
Overlord Momus is an idea created by Arfoire in order to spread fear amongst the people and thus grant Arfoire more power.
It seems that Momus did not actually exist as a monster in the original 2010 game (only as an idea), but does in the 2013 remake Hyperdimension Neptunia Re;birth1; additionally, the Momus you see in the remake is the same form as a form that Arfoire herself takes in the 2011 game Neptunia mk2.
Overlord Momus is one of the largest (if not THE largest; the only competition are the Dangerous Foes Alnair and Herobo) and ugliest monsters in the game, and you cannot see its top half during battle except for the first second or two when the camera is still zooming in. To further cement the notion that Momus is a monster rather than an actual character, it appears in a stronger(!) form as one of the Dangerous Foes you can add to the Fantasy Zone dungeon where it originally appears, in the name of Viral Mareshimono.