The kaijin utilized by Geddon are called Beastmen, grotesque bestial beings created by Gorgos. Having the strength of beasts with human intelligence, the Beastmen are conditioned to defeat the enemy even if they die in the process and such actions like retreating or failure are punishable by death and then are cooked for Gorgos to feed on.
Spider Beastman
A spider monster able to regenerate his limbs, Kumo is the first of Geddon's Beastmen to pursue Daisuke from the Amazon to Japan where he kidnaps people with his deadly webbing and spider familiars. When Daisuke arrives, Kumo kills Matsuyama before wounding Kousaka as he and Daisuke escape. Finding Daisuke, Kumo kills Kousaka before going after Ritsuka and Masahiko. Infuriated, Daisuke attacks Kumo with extreme prejudice, ripping off the monster's limbs before he falls back. Returning to his master, Gorgos incinerates him for his failure.
Episode 1
Beastman Vampire Bat
A Bat monster sent after Amazon, Vampire Koumori overpowers him with his flyng abilities until Amazon drives him off. However, before Vampire Koumori fell back, he bit Masako as she suffers a horrific fever beyond the knowledge of medical science. After feeding on a man to regain his strength upon getting a second chance, Vampire Koumori has the Jusha lure Amazon into a cave where he has the advantage. However, Amazon heavily wounds the Beastman before crippling one of his wings. Running off, Vampire Koumori is killed by Gorgos for putting his life above Geddon's goals, devoured by vampire bats as a result.
Episode 2
Mantis Beastman
Newly created by Gorgos, Kamakiri is sent to attack humans with his petrifying sailva in hopes that Amazon would show up so he can take the GiGi Armlet from him. He attacked Tobei when Amazon intervened, defeated by the monster. Kamakiri later attacks Amazon when he save Masahiko from a pair of Akajusha. In the end, Amazon causes the monster to be coated with its own saliva, turned to stone as he goes out to sea.
Episode 3
Beastman Big Centipede
A Beastman who is sent with the Superior Geddon Bike Team to go after Yamamura, a member of Kosaka's exploration party into the Amazon who knows the existence of the GaGa Armlet. After finding Yamamura, Omukade captures them with Daisuke in pursuit on the Jungler. After having Amazon bite off one of feelers and knocked over the edge of a cliff, Omukade is killed by Amazon's Big Slice.
Episode 4
Beastman Porcupine
A Beastman sent to kill Amazon and Mogura, Yamaarashi can turn himself into a spiked ball. He attacks Daisuke with he and Masahiko escaping the Beastman. Yamaarashi later steals the Quipu from Jouhouku University as it had clues to Geddon's agenda, provoking Daisuke into a trap. With Mogura's aid, Daisuke regains the Quipu before he battles Yamaarashi, killing him with his Big Slice as the Beastman bleeds out blue blood.
Episode 6
Snake Beastman
One of the most firece of Geddon's Beastmen, the Hebi is able to extend his body and use his tail as a weapon or to contrict his prey. He is sent to after Daisuke through Hiromi Arima and her biologist father, Professor Seichiro Arima. After eating the family friend, Izaki, Hebi creates a clone of his victim to have Professor Arima meet Daisuke and capture for the sake of science. With Daisuke captured, the clone melts as Hebi reveals himself. After Tobi and Masahiko free Daisuke, he drives off the Beastman as the children are taken by the Akajusha. Daisuke pursues them to an amusement park where Hebi has the advantage. But once assuming rider form, Amazon overwhelms Hebi with blows to the head before using his Big Slice to rip the Beastman's snout in two.
Episode 7
Crocodile Beastman
A Beastman who sneaks into Jonan Elementary, posing a stuffed animal while feeding on a janitor as Masahiko and his classmate Misuru witnessed it. The monster than proceeds to cause an evacuation in order flush out children for the Akajusha to capture and sent to Mt. Suna as part of Geddon's plan to eat the children after steaming them alive. Given a chance to redeem himself for failing to get the GiGi Armlet, the Wani attacks after Daisuke is tied up by the angry parents. Assuming his rider form, Amazon makes it to Mt. Suna to save the children after he takes a bite of Wani's face and uses his Big Slice on the monster's brainpan.
Episode 8
Crab Beastman
Awakened by Gorgos, with the ability to control man-eating crabs and sprew acid from his mouth, Kani is sent to gather human women so Gorgos can feed on their blood. Promised a raise in the ranks, Kani has the Akajusha capture a woman and lure Daisuke to him. After losing to Amazon, Kani captures Mogura and interrogates him to gain information on Amazon's weakness, targeting Ritsuko as a result. Catching Ritsuko and her friend Sayo after Tobei attempts to protect them, Kani takes the girls to his island with intent to friend them to his man-eating crabs as Amazon pursues them. Using the girls, Kani overpowers Amazon until he assumes Rider form and uses his Big Slice twice to kill the Beastman as he falls and dissolves into a pool of orange slime.
Episode 9
Black Cat Beastman
A black cat monster with poisonous claws. Attacking Daisuke in the middle of night, Black Cat manages to poison him before he escaped. While Tobei goes off with Masahiko to find his medical herbs, Daisuke is placed at the Saint Maria Nursery School. Learning of Amazon's location, Black Cat makes his way to St. Maria and attacks Daisuke who uses his Condora to drive the cat off by blinding his right eye. Vowing revenge, Black Cat has the Akajuusha takes the child Kazu as their hostage. Though he saves Kazu, Daisuke is attacked by the Beastman. Assuming Rider form, Amazon battles Black Cat until he uses his Big Slice to decaptiate the Beastman, knocking his head over the distance.
Episode 10
Beastman Katatsumuri
A snail Beastman sent after Amazon to take advantage of Masahiko's habit of collecting snails, Katatsumuri poses as a golden-shelled snail which an Akajusha placed in the boy's snail jar. Kat makes his move after school, chasing after Masahiko and his friends with his slime before Amazon arrives and takes their fight away from the children as the Akajusha spirit them off. Outmatched, the Beastman is knocked off into the river. Katatsumuri later appears as Daisuke wisen up to the Akajushas' decoy plan. In the ensuring fight, after Tobei help him, Amazon did enough damage to the Beastman for him to retract into his shell as Amazon flings it into the river where it explodes.
Episode 11
Togeari Beastman
He was originally Genji Murata, a murderous criminal who Daisuke saved a girl from when he used her as a hostage. On his way to jail, he was taken by the Akajusha and brought before Gorgos who offers to make him a BeastMan for the evil he committed. Accepting it over the death penalty, Genji is converted into an ant Beastman with his mind altered and able to shoot a flammable liquid from his mouth. Togeari is overpowers Daisuke with aid from an Akajusha's aid as they bring him to Gorgos. But when it turned out to be Daisuke's plan to get the GaGa Armlet from Gorgos, Amazon is forced to escape Geddon's collapsing base as he and Togeari battle outside. Fighting at full strength, Amazon uses his Big Slice to knock the Beastman's head off after he attempted to take him down with him.
Episode 12

Beastman Hebitonbo
A dobsonfly Beastman who attacks Masahiko and Miri, a woman who joined them in their fight against Geddon, when Amazon arrives with Mogura covering their escape. Hebitonbo later attacks Mogura before spiriting Miri away and use he to keep him from Gorgo's safe house and get the GiGi Bracelet. But once Miri is exposed as an Akajusha, Hebitonbo batttles Amazon and has his arms ripped off. Refusing to reveal Gorgos's hiding place, Hebitonbo pupulates himself into a coccon laced with Hira-inya. The Kurojusha steal it before the Akajusha can retrieve Hebitonbo and place it in the basement of a hospital before being brought to Garanda's base where Hebitonbo emerges in his imago form, astonished to find himself before Emperor Zero and offered a place in Garanda Empire if he takes the GaGa Armlet from Gorgos. Using Masahiko and Yumi as offerings to Gorgos to get the bracelet, Dobson encounters Amazon and battles him until he convinces Amazon to team up with him. But during the fight, Hebitonbo attempts to get GiGi Bracelet as well and is cut to pieces by Amazon's Big Slice.
Episodes 13-14
Series:  Kamen Rider Amazon