|Home world:||Mahagera Village|
|First appearance:||Ultraman Cosmos Episode 36: "The Mountain of Yōkai"|
|Weight:||64 000 t|
Subtitle: Legendary Fiend (伝説悪鬼 Densetsu akki?)
- Height: 63 m
- Weight: 64 000 t
- Origin: Hidaka Village
One day, Mahagenom rampaged on an old folks home. Team EYES were dispatched where they tried to transquilize the monster but failed. After assaulted by JADF tanks, he ran away into hiding at the mountains of the Hidaka Village. When the monster started it's assault again, Yamawarawa appeared and battle the demon as the villagers cheer for the protector. It was revealed according to the legend, Yamawarawa had once fought the demon Mahagenom and brought peace to the Hidaka village. Cosmos appeared and aided Yamawarawa, giving him some time to charge his greatest attack to seal Mahagenom forever. Yamawarawa bids farewell to the children and goes on a journey back to his old village.
- Suit actor: Toshio Miyake
- Mahagenom's roars were recycled from Golza.
- Mahagenom's suit would later modified for Lesser Bogar.
- Mahagenom's design is in tribute to Ashuran.
Powers and Abilities
- Durability: Mahagenom can resist the poison/pain unleashed by Team EYES' transquilizer missile.