The Mu Empire is ruled by the supernatural Grand Creator of Mu [Mu Daitei].
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.
Overseeing the Empire for Grand Creator of Mu are Crown Prince Gill & Black Mask.
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 04: Oil Primitive [Oiru Genjin]
Episode 08: Stone Primitive [Ishi Genjin] (killed Takigawa, Go's girlfriend)
Episode 13: Toad Primitive Rotund Toad [Gama Genjin Gamatsubura] (haunts Aoki, Jiro's dreams)
Episode 17: Earthquake Primitive [Jishin Genjin] & Ocean Priest Primitive [UmiBozu Genjin]
Episode 22: Dog Primitive [Inu Genjin] (an honorable swordsman, hates humans for "enslaving" dogs)
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 Footmen are super strong and 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.