Mu Empire
The Mu Empire is ruled by the supernatural Grand Creator of Mu [Mu Daitei].
The Grand Creator of Mu hides its true form behind a gaint stone statue, probably designed to terrorize its followers. The actual creature is a monsterous brain kept alive in a glass case. Although it immobile, cannot speak (apparently its telepathic) and appears helpless, the Grand Creator of Mu is in fact extremely dangerous as it can generate a deadly laser beam, which it fires through the eyes of the statue.
Overseeing the Empire for Grand Creator of Mu are Crown Prince Gill & Black Mask.
Like their master Crown Prince Gill [Giru Taishi] (episodes 1-26) and Black Mask [Kuro Kamen] (episodes 1-25) hide their true forms. Crown Prince Gill's true form, that of a hideous mummy, is revealed in the very first episode. While Black Mask's true form is never seen.
Mu Empire monsters are all "genjin", which means "primitive man", but because they are not human I dropped "man" from their names. Some monsters make brief appearances in more than one episode, for example the first 8 all appear in the first episode, however I'm only listing the episodes in which each monster has a major role. Like their Grand Creator most Mu Primitives don't talk, the few who do speak say very little.
Episode 01: Sand Primitive Sand Grudge [Suna Genjin Sunaurami]
Episode 01: Magma Primitive Flame Man [Maguma Genjin Kaenjin]
Episode 01: Water Goblin Primitive [Kappa Genjin]
Episode 02: Mummy Primitive [Miira Genjin]
Episode 03: Amphibian Primitive Amazon X [Hangyo Genjin Amazon X]
Episode 04: Oil Primitive [Oiru Genjin]
Episode 05: Snake Primitive [Hebi Genjin]
Episode 06: Electricity Primitive [Ereki Genjin]
Episode 07: Wolf Primitive [Ookami Genjin] (female white wolf monster is assisted by 4 weaker males)
Episode 08: Stone Primitive [Ishi Genjin] (killed Takigawa, Go's girlfriend)
Episode 09: Flying Dragon Primitive [Hiryuu Genjin]
Episode 10: Child of the Earth (Soil) Primitive [Tsuchinoko Genjin]
Episode 11: Wax Primitive [Rou Genjin]
Episode 12: Fly Primitive [Hae Genjin] (Hayashi, Sanpei was turned into this monster)
Episode 13: Toad Primitive Rotund Toad [Gama Genjin Gamatsubura] (haunts Aoki, Jiro's dreams)
Episode 14: Skull Primitive [Dokuro Genjin]
Episode 15: Gas Primitive [Gasu Genjin] & Spider Primitive [Kumo Genjin]
Episode 16: Spider Primitive
Episode 17: Earthquake Primitive [Jishin Genjin] & Ocean Priest Primitive [UmiBozu Genjin]
Episode 18: Earthquake Primitive & Ocean Priest Primitive (strongest and weakest monsters)
Episode 19: Meteor Primitive [Ryuusei Genjin]
Episode 20: Rat Primitive [Nezumi Genjin] (hates all who harm rats, including Mu leaders)
Episode 21: Bagworm Primitive [Minomushi Genjin]
Episode 22: Dog Primitive [Inu Genjin] (an honorable swordsman, hates humans for "enslaving" dogs)
Episode 23: Coal-Tar Primitive [Kooru Taaru Genjin]
Episode 24: Plant Primitive [Shokubutsu Genjin]
Episode 25: Marionette Primitive [Marionetto Genjin] & Black Mask
Episode 26: Water Buffalo Primitive [Suigyuu Genjin] & Crown Prince Gill
The Mu Empire's army consists mostly of the fast moving Mu Footmen [Mu Batei] Although they appear at the beginning they don't see any action until episode 12. The Mu Footmen sometimes disguise themselves as humans. Although capable of moving faster than an automobile and possessing super strength they are not bullet proof. When the Footmen are killed their bodies vanish.
Monster names romaji and English translations by Steve Okubo. Photos from Japan Hero.