In the episode, Tuddrussel had ignorantly squished a fly while the team was in the Stone Age. The team discovers that this led the species to overcompensate as it further evolved, so it evolved into a fire-breathing monster by the medieval ages.
The fly devours all of the knights in the kingdom, but when it attacks Tuddrussel, it just carries him around for several hours. Tuddrussel eventually gets bored of it and blows it up with a grenade, although this allows the species to evolve enough to build the Fly City.
The Fly is enormous, with massive legs, wings, and green eyes. It has small spikes along its back.