Ninjabot
Ninjabot resembles a samurai more than a ninja and is armed with a katana that could shoot energy bolts. He was able to cause the Galactic Sentinels' weapons to disappear. The Galactic Sentinels managed to knock Ninjabot down causing Gorganus to fire a beam that revitalized Ninjabot. He was destroyed when they formed Knighttron for the first time.

In "The Spy," Ninjabot appears in the desert. Gordon is sent to the desert and ends up landing an attack on Ninjabot. Even though Gordon seemingly destroyed him, he learned from Nimbar that Ninjabot wasn't actually there. Ninjabot speaks to Gorganus in "Turncoat," he pledges loyalty to him but knocks him out. He takes Taurus to tell him they can destroy Gorganus, he will take over and leave Earth. Ninjabot even capable of treachery, he can't lie. The Sentinels choose to just destroy Ninjabot.
"In The Beginning," "The Spy", "Turncoat", and"Winner Takes All"

The Sorcerer
A masked wizard that can use magical attacks and can teleport. He was sent by Gorganus to disrupt the 24 Hour Time Cycle. Time kept speeding so that the Galactic Sentinels had a hard time getting through a Power Portal. The Galactic Sentinels end up forming Knighttron who defeats the Sorcerer. Time went back to normal.

In "Mind Games," the Sorcerer suddenly retreats leaving his wand behind which ends up causing Nimbar to fall under the Sorcerer's mind-control to make Nimbar disband the Sentinels. Using Swinton's science project, the teens return to Nimbar's lair and throws the Sorcerer's wand away. The Sorcerer returns at full strength and the Sentienls fight the Sorcerer.

In "A Nightmare on Rodeo Drive," the Sorcerer is sent to make the teen's nightmares come true. When alerted about the Sorcerer's attack by Nimbar, the Sentinels end up fighting the Sorcerer who ends up overwhelming them enough for them to retreat. After a series of unlucky events after that, the teens discover that they are in a dream spell caused by the Sorcerer. When they wake up, they continued their fight against the Sorcerer. Before the Sorcerer can cast another spell, they form Knighttron and defeat the Sorcerer.

In "The Leech," the Sorcerer places a Mind Leech on Drew so that the Mind Leech can give Emperor Gorganus the true identity of the Galactic Sentinels.

In "The Ghost Warrior," the Sorcerer is sent to Earth disguised as Taurus in a plot to make a public appearance to lure out the Galactic Sentinels. Nimbar figures out the Sorcerer's plot and the teens engage the Sorcerer. The Sorcerer retreats after a hit from Taurus in order to further their plot. The Sorcerer attacks Earth again causing the sentinels to fight the Sorcerer with Gordon's mother watching. The sentinels defeat the Sorcerer who is then withdrawn by Emperor Gorganus. When the Impostor Taurus disappears, Emperor Gorganus sends the Sorcerer to the Toil Mines.
"How Time Flies," "Mind Games," "A Nightmare on Rodeo Drive," "The Ghost Warrior" and "The Leech"

Slaygar the Toxic Waste Monster
A sludge waste monster who appeared in "Perceptions." He was sent to wipe out the human race by poisoning the planet with the toxic fumes he breathed. His pollution damaged the power portals and only Apollo was able to transform and fight. Apollo learned from Nimbar that Slaygar's weak spot was behind his ear. Apollo eventually discovered Slaygar's ear was actually in his stomach and began to win, but Emperor Gorganus recalled Slaygar before he could be destroyed.
"Perceptions," "Ozone O-Mio", "Opiate from the Future", and "The Primal Scream"
Neuragula
A brain monster who first appeared in "The Note." It has two hemispheres: one controlling emotions the other controlling intelligence. Its most distinguishing ability would be its ear-piercing screech. It could shoot laser beams, shoot energy prisons, and could mind control its victims. Its main weakness was having to force himself to use another part of his brain.

In "The Quitter," he controlled Laurie with one hemisphere. He made Laurie against time and she quit the team. The Sentinels figured out if they danced, they would confuse him and loosen the hold he had on her.

In "Brain Drain," Neuragula drains everyone's brain drains and stores them in a crystal. Gordon is invulnerable thanks to Swinton's wave-blocking helmet and when he touches the crystal, he gains ultimate intelligence. Neuragula tries swaying Taurus to the dark side by defeating his friends but Taurus hits them with the crystal to restore their intelligence and defeat Neutagula.

In "Deja Vu," Gorganus made Neuragula have the power to make the Sentinels repeat the same battle over and over, as an result they repeated the same day over and over.
"The Note," "The Quitter," "Brain Drain", "Deja Vu", and"Penny for your Thoughts"

Voldek
An electrical knight monster that could produce static electricity to disrupt the power portals in the episode "Switch" which caused the Galactic Sentinels' minds to be in different bodies.

In battle his sword could shoot lightning and proved a formidable foe until the Galactic Sentinels formed Knighttron and destroyed Voldek by cutting one of his shoulder cords, thus short-circuiting him and putting the Galactic Sentinels' brains back into their respective bodies.

In "Trust," Emperor Gorganus modified Voldek with a power receptacle pack and sent him to take out Earth's electrical grid. With Voldek's attacks on the same frequency as the Galactic Sentinel's weapons, they had to use a code that they remembered in order to take out the the power receptacle pack. After the Galactic Sentinels destroyed the power receptacle pack, Emperor Gorganus recalled Voldek.
"The Switch," "Trust", "The Impostor", "The Glitch," and "The Last People On Earth"

Octodroid
Octodroid is an octopus-themed monster. He has the ability to turn freshwater into saltwater and drain the moisture from others. Emperor Gorganus sends Octodroid to turn the freshwater into saltwater. During the fight between the Octodroid, Drew's essay was eaten by Octodroid. When the Galactic Sentinels turn the tides against Octodroid, Emperor Gorganus withdraws Octodroid. Octodroid returned and attacked the Nuclear Power Plant in order to drain it of it's cooling water. During the second fight, Octodroid starts to drain the moisture from the Sentinels. The team had to form Knighttron who ends up caught in his tentacles. Knighttron breaks free and Octodroid is withdrawn by Emperor Gorganus.

In "The Monster Among Us," he steals a bit of skin from Gordon and makes a clone that is a robot of Taurus. Octodroid freezes the real Taurus. When the evil Taurus is found out, Taurus is unfrozen and Octodroid is destroyed again.
"Three Cheats to the Wind", "The Cover-Up", "Commitments", and "The Monster Among Us"

Isolus
A female ice-elemental monster who uses ice attacks, wields a sword and shield.

In "The Spy," Isolus is sent to Earth and disguised herself as a foreign exchange student named Ilse in order to learn the true identity of the Galactic Sentinels. Reprogramming the power portal, Ilse ends up in the desert and transforms into Isolus. The Galactic Sentinels head to the desert and battle Isolus. She manages to freeze Gordon who is thawed by Swinton. The Galactic Sentinels form Knighttron who deflects Isolus' ice attack back at her. Emperor Gorganus plans to reconstruct her while using her residue for his drink.

In "The Y Files," Emperor Gorganus sends Isolus to Earth's desert in order to start a new Ice Age. When Nimbar detects this, he sends the Sentinels. Isolus returns to Earth and fights the Galactic Sentinels where Isolus ends up freezing Apollo enough for the Sentinels to retreat back to Nimbar.

In their next battle, Isolus makes their weapons disappear, the Sentinels form Knighttron and fight Nitghttron. After disarming Isolus, Knighttron ends up deflecting Isolus's ice attack back at her. With a new chip Gorganus added in her in "Take Two Galatical Sentinels and call Nimbar in the Morning," she was able to alter the Earth's climate. Gordon got sick and so did Nimbar and Drew. Taurus had to fight her to get a special bark to get Nimbar and everyone feeling better. When they were better, they defeat Isolus one last time.
"The Spy", "The Y Files", "Gordon Cries Wolf" and "Take Two Galatical Sentinels and call Nimbar in the Morning."

Culebra
A snake-like monster who can breathe heat.

In "The Rat," Culebra gains the ability to raise Earth's heat. When the Galactic Sentinels attack Culebra when the sun sets, Emperor Gorganus recalls Culebra. Emperor Gorganus modifies Culebra with a power retainer and sends him back to Earth. Due to Gordon ending up in the body of Swinton's rat, the others had to fight Culebra without him while Nimbar found a way to undo what happened to Gordon. Once Nimbar found a way to get Gordon back to normal, Gordon joined up with the other Galactic Sentinels to help fight Culebra. The Galactic Sentinels weren't able to form Knighttron until Gordon teleported by to Knighttron and got Darwin off the teleporter pad. Once this was done and Knighttron was formed, Culebra was defeated and withdrawn by Emperor Gorganus.

In "Emperor for a Day," Gorganus puts Lechner in charge and he sends Culebra to attack and pulls him back and sends him back later. This almost defeats the Sentinels until Gorganus returns. He can make fireballs from his mouth.
"The Rat", "Bully for you", "The Chocolate War", and "Emperor for a Day"

Predaraptor
A raptor-like monster is nearly indestructible. He creates a light above the ridge of his head before he emits a deadly beam. He destroyed previous sentinels Ursa, Leo and Draco, leaving Rick/Orion alone. When Predaraptor injured Laurie, making her ears hurt. Orion joined the others in battle in place of Scorpio and suggested to hit Predaraptor in the back and was able to defeat him.
"The Universal Hitchhiker", "Mr. Popularity", and "Beverly Hills 902-Oblivion"
Snake Trooper
When we first see this alien, the Sentinels defeat him with a move Swinton did he learned from Drew. Gorganus chastises him but he puts a chip in Snake Trooper to learn the Sentinels' every move. Taurus decides to do the same thing they did last time but they miss and Snake Trooper beats the Sentinels up. They form Knighttron and send him back to Gorganus. Gorganus then programs him to be able to anticipate Knighttron's every move. The second time he is sent, they use Knightron and Snake Trooper avoid the moves. Gorganus sends his sword to Snake Trooper and it takes down Knightron. Knightron quickly gets its sword and shield again and teleports away. Snake Trooper returns and the Sentinels put their weapons down and try talking due to a psychiatrist's suggestion. They get blasted and form Knightron. They are defeated again and teleport away. The last time they meet they say they will form Knightron but they don't. They destroy him with their weapons without Knightron. Snake Trooper is the only monster that appears twice and before the last episode.
"The Psychiatrist - Part 1 and 2"
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