Strange Demon
The monsters used by the Crisis Empire are referred to as Strange Demon Soldiers. There are four branches each lead by an Officer. While Granzyrus appears in Earth, Maribaron uses her power to revive members of the each other armies into the Spirit World Kaijin Army, being greyed versions of themselves before being destroyed by BioRider.
Strongest Kaijin Granzyrus
A bird-like monster which was the Grand Lord Crisis' best soldier and worked with Bosgan when summoned to Earth. While the 10 veteran Kamen Rider team fought the Chaps, after killing Bosgan, RX became BioRider and destroyed the monster from the inside out. Appeared in Episodes 44 and 45.
Strange Demon Supernatural Clan
The Strange Demon Supernatural Clan unit is used by Colonel Maribaron.
A Grim-Reaper style monster with a scythe and assisted by two hornless versions of himself, Skullmastars. He played a part in Kotaro's rebirth into Black RX, killed by him while the Skullmastars live on to serve the Crisis Empire on different occasions. Skullma returned in the Kamen Rider Black RX movie to fight Kotaro before he was sucked into a dimensional warp.
Episodes 1-2, 45 and Movie
Two hornless versions of Skullma.
Episodes 1-2, 9, 19, 32, 42 and Movie
A silver armored swordsman with ability to harness the sun's rays and use them a weapon. He could also move through anything that's reflective. His mission was to steal something from Yousuke Morita, an old friend of Kotaro's. After a brief fight with Bujin, Kotaro learns the blueprints are for a laser the Crisis Empire wants to build. To get the blueprints from Kotaro, Bujin uses Yousuke as a hostage and tricks Kotaro into giving him the blueprints. From there, RX battles Bujin until used his Revolcane to destroy him.
Episode 7
An elf-like monster who wielded a blade and could shoot a beam from his forehead crystal. Maribaron used Zunojin to attack Reiko in her dreams, taking control of the girl's subconscious in another plan to target Kotaro's weaknesses. With the aid of a dying Karu, RX uses the Crisis Marine's machine to enter Reiko's dream, giving Reiko back control over her dream before killing Zunojin with his Revolcane after a brief hestitation.
Episode 12 & 45
A green and white-furred shaman with bat wings for ears. It is armed with a skull-headed staff and bananas that can be used as weapons as well as for other purposes. Byakki used his bananas to infiltrate a gold depository to steal the gold. Joe told Kotaro about a weird old man who bought bananas from his banana stand after following him to his house. Shigeru and his friends stumbled onto his house and were captured. When Kotaro arrived and found Shigeru and his friends, the weird old man revealed his true form of Bya Fiend and attacked Kotaro. Byakki threw a banana into Kotaro's chest knocking him out and then kidnapped him. Atop a building, Byakki tried to use a big banana to push the impaled banana further into his chest, until Kotaro used his Robo Rider ability, which caused the banana to break and allowed him to recover. Kotaro transformed into Kamen Rider Black RX. After some banana attacks from Bya Fiend, Kamen Rider Black RX became Bio Rider. While on Mach Jabba, Bio Rider attacked Byakki and used his liquid powers to evade his banana attacks. Bio Rider broke Bya Fiend's staff before destroying him with his Bio Blade.
Episode 20
A plant-covered rock monster who wore an orange loin cloth. He can manipulate soil, split into rocks, and launch the rocks on his body. His stone heart can allow him to regenerate from Black RX's attacks. Maribaron sent Ganma to battle RX as a diversion while he used a girl to restore a key to the demon world for their invasion. When Kotaro was ambushed and pulled underground by Ganma, he became Kamen Rider Black RX and then became Bio Rider which allowed him to escape to the surface. He then reverted to Kamen Rider Black RX and fought Ganma before pulling back to find Maribaron. After RX thwarted Maribaron, she used Ganma to fight him. During the battle, RX discovered Ganma's stone heart and called out his Revolcane which he threw at Ganma's stone heart before fully impaling it, which led to Ganma's destruction.
Episode 32
A one-horned bluish imp monster armed with a pitchfork. He can shoot a beam from its horn. Maribaron planted him inside Kotaro to kill him when he was being pursued by the police. This was a plan by Maribaron to hack into the city records and make specific people look like criminals. When Kotaro found out about Wilki while he was in jail, he henshined into Kamen Rider Black RX and escaped from jail. Wilki even poked him while he was escaping on Acrobattler. While in the sewers, Kamen Rider Black RX became Bio Rider and found out what the Crisis Empire was up to. When the next person was arrested, Wilki attacked Kamen Rider Black RX and attempted to deliver the final blow. Kamen Rider Black RX faked his death and was brought to Maribaron. He eventually became BioRider right as Maribaron was about to destroy him. BioRider revealed that his form prevented Wilki from killing him. With all the police computers experiencing a system down, Maribaron retreated and Wilki attacked. BioRider called out his Bio Blade and fought Wilki and the Chaps. BioRider managed to chop off its horn and destroy its pitchfork. BioRider then reverted to Kamen Rider Black RX and brought out his Revolcane to disarm Wilki and destroy him.
Episode 35

She is a powerful fortune teller and miracle healer that lured people to help build a city for Crisis. She is actually an ape-like beast beneath her human appearance. She was killed by RX's Revolcane.
Episode 38
Hundred-Eyed Hag
She is Maribaron's aunt and a somewhat very powerful witch. She absorbed Kyoko into herself and later Kotaro and Joe. However Kotaro transformed into Biorider and back to RX and defeated her.
Episode 41
Series:  Kamen Rider Black RX