Monster Army
The Monster Army is composed of genetically altered engineered mutants. After being killed, they tend to melt into a puddle of green slime:

Victorious Saint Geldring
A green muscular beast with a skull-like head encased in a clear egg-shaped dome that resembles a mutant turtle. Geldring's other abilities include teleportation, shooting goo out of his fingers, producing electricity from his hands, breathing fire and releasing vine-like appendages from his mouth. He has the most opportunities to battle Metalder personally, but fails. His personality is dirty, treacherous, envious, cowardly, and accommodating. He frequently criticizes the other armies and often takes credit for work accomplished by his fellow mutants. In Episode 9, he teases and taunts Hedogross for his love of Wisdom and soon prompts the other mutants to do the same. In Episode 15, despite Hedogross Jr. fitting into his rank, he does not want the young mutant to represent the Monster Army. In Episode 32, he learns of a rare plant which enhances those who consume it with great power. He puts this newfound strength to use against Metalder, but does not realize the effect is temporary. He is presumably destroyed after this fight, by Metalder, when Metalder uses his Laser Arm. In this battle, Geldring reveals the ability to blast lasers from his eyes as well as breathe fire. Geldring resurfaces in Episode 35, however, to battle alongside Bankora and is finally destroyed in this match, again by Metalder's Laser Arm attack, turning into a puddle of slime.

Brave General Bridy
A white furred wolf creature who ains a detachable spider-like parasite on his back. Bridy is extremely ferocious and uses his claws and fangs to attack Metalder. Bridy can shoot webbing to bind his victims. He is among the monsters sent to capture Top Gunder. In Episode 15, he presents Wisdom as a hostage as well as leading the Monster Army and the Shadow Light Fighters into fighting Metalder and Hedogross Jr. In Episode 19, he fought Metalder and Springer in the Ghost Bank. At one point during that battle, Springer bites him, once freed of imprisonment. He battles and bites Metalder in Episode 23, frying his circuitry. During that same episode, he participates in the Four Army relay race, which the Monster Army wins through heavy cheating. In Episode 28 he reveals he can launch his claws like sharp missiles as well as shapeshift into a humanoid form via the power of a special diamond. He was thought defeated in Episode 28, melting into a puddle of slime. Restored, he appears in Episode 32. He appears with his parasite in Episode 34 when Metalder learns of this weakness and kills the parasite during their final battle with his Laser Arm attack. This kills Bridy and he dissolves into slime.

Violent Spirit Bankora
A grey-skinned scorpion-like monster with white fangs on his chest and legs, and a third arm on his back. He can spew acidic saliva from his mouth and he can teleport. In Episode 5, he is chosen by Geldring to battle Top Gunder in the Ghost Bank. He loses, but survives by forfeiting. He is one of the monsters sent to capture Top Gunder. He later appeared to snag a hostage for Hedogross. He fights Metalder and Springer in the Ghost Bank, only to be viciously attacked and bitten by Springer. He competes in the Four Army relay race in Episode 23. During the battle in Episode 35, he fights alongside Geldring. He is eventually destroyed for good when Metalder binds him with his own arm and then sends him flying with his G-Kick, causing him to collapse and explode.

Heroic Fighter Gamadone
A brown-skinned monster with antennae and tentacled arms that electrocute enemies. He can also produce green slime on his tongue and spit it at opponents, as well as turning his right hand into a crab-like claw and breathing fire. In Episode 4, he is injured by Metalder when the hero yanks out one of his internal organs and knocks him into the water. He survives and is seen fighting Hedogross Jr. in the Ghost Bank in Episode 15. In Episode 17, he kidnaps Mai to lure Metalder to him and tries to make love to her. He then battles Metalder again and is destroyed by his Laser Arm attack. Gamadone appears as an extra in the Metalder movie.

Explosive Fighter Damnen
A red-eyed lizard-like mutant. He fights Gochak for the privilege of fighting Metalder in Episode 2, but loses when his head was twisted backwards by Gochak. Battling Gochak, Damnen reveals he can blast lightning-bolt like lasers from his eyes. Even though he loses to Gochak, he survives. He appears in Episode 5 with Zakembor to spy on Top Gunder. Both are caught and punished by Valsky and Geldring. He is one of the monsters hunting Top Gunder. In Episode 8 he helps Hedogross fight Metalder. Damnen is badly injured fighting Hedogross Jr. in the Ghost Bank in Episode 15. In Episode 19, he fights Metalder in the Ghost Bank. In Episode 32, he appears alongside Geldring in the hunt for the rare plant. He is destroyed when Metalder kicks him off a cliff and explodes after landing at the bottom of the quarry.

Furious Fighter Zakembor
A red-eyed cicada-like mutant. He can emit sonic waves by flapping his wings and spit a green, acid-like liquid. He accompanies Chuubo who kills Dr. Koga and first fights Metalder. He and Damnen sneak away to spy on Top Gunder in Episode 5. Both are caught and punished by Valsky and Geldring for sneaking off. In Episode 8, he helps Hedogross battle Metalder. Zakembor fights Hedogross Jr. in the Ghost Bank in Episode 15, and is badly injured. Later, he fights Metalder and Hedogross Jr. again and is destroyed by the latter. Zakembor was an extra in the Metalder movie.

Light Fighter Hedogross
A green frog-like mutant who took a female slave, Wisdom, as his wife. Hedogross can dissolve into green slime and first appears in the Ghost Bank arm wrestling with the Medium Fighters and repair robots. In Episode 8, he fights Barlock in the Ghost Bank and wins the chance to fight Metalder. With the help of Bankora, he takes a scientist hostage. Barlock sneaks in on him and knock him out, swiping the hostage in the process. His fellow mutants, including Geldring, mock him for his love of Wisdom. He fantasizes about being promoted in the Empire so that he can both love Wisdom and be a respected and powerful warrior. God Neros promises a promotion, and that his dreams would come true, if he destroys Metalder. However, he dies first. Battling Metalder, he damages his armor. Darbarbo and Bulchek seem to cause interference in the battle. Metalder's combined Laser Arm and G-Kick attacks cause him to fall over and explosde. Before dying, Hedogross spews liquid all over himself and dissolves into slime. Sometime before his death, he conceives a son with his wife Wisdom, who would be named after him.

A slave married to Hedogross, who later gives birth to Hedogross Jr. She has both a humanoid and a mutant form. When she first appears, in Episode 9, she gives her husband her earring to hold during battle. When he is destroyed, she plucks the earring from his remains and vows revenge on Metalder, getting her chance in Episode 12. She uses her human guise to portray a damsel in distress for Metalder to rescue before revealing her true form and attacking. She flees when her labor pains become too intense. Back at the lab, Metalder learns she is just a pregnant slave. He saves her from Galador, who attacked her because she failed to defeat Metalder and ruined their plan. After Metalder rescues her, he returns her earring which she dropped fighting him, and allows her to escape. Sometime before Episode 15, she bears Hedogross Jr. When Junior attacks Metalder, she becomes angry and informs him that Metalder had saved her life. She tries to steer her son away from the Empire, but in vain. Wisdom is later taken hostage by the Empire, but is rescued by her son. She helps both he and Metalder fight off the Empire before leaving the Neros Empire with him.

Hedogross Jr.
Hedogross Jr. is the son of Hedogross and Wisdom. Junior is a green mutant with silver hair who could project three long claws on each hand. Like his father, Junior can dissolve into slime and takes human form like his mother. He first appears in Episode 12 in Wisdom's memory/imagination as a baby. In Episode 15 he is determined to exact revenge on Metalder for his father and wants to take his father's place. Junior is recruited by Galdos and given status after proving himself. He is assigned to kill Metalder by God Neros after thrashing numerous members of the Monster Army in the Ghost Bank. However, during mid-battle with the Metal Hero, Metalder saves him from falling off a cliff. Metalder tries to reason with him. After seeing his mother held hostage, Junior finally realizes God Neros is the true villain, and the two fight off Bridy, Gamadone, Damnen, Zakembor and the Shadow Light Fighters together.

Dog-Gyaran is the end result of Geldring's attempt to revitalize his ranks. Originally a stray German Shepherd Dog named George, he is captured by God Neros's secretaries and mutated into a member of the Monster Army via a special booth-looking machine. During the battle with Metalder, its memories are restored by the Saint Bernard belonging to Mai and Kita, who had earlier shown it kindness. When it refuses to follow orders and destroy Metalder, Geldring kills him by spraying him with toxic foam.