Allow me to introduce you to the kaijin who appeared so far in the new show,
"Kamen Rider Decade"
N-Gamio-Zeda is the king of the Grongi Tribe in Kamen Rider Decade, also called "The Ultimate Darkness". He refers to himself as otherworldly because he should have never been revived. He is sealed and the Grongi are attempting to unseal him with a Sacred Gegeru, targeting five police women without spilling blood to revive him. Though they obtain four kills, Tsukasa uses Ai as bait to lure the Grongi out and gives her a nose bleed to ruin the Gegeru. The effects of the distortion revive Gamio anyways, and he uses his power to convert every human corpse within his Miasmic aura into a Grongi. Although his ultimate goal is to have his sired Grongi kill each other, Gamio is destroyed by Kuuga and Decade performing a combo attack.


The previous King of the Fangire Race, the
Beetle Fangire was exiled for loving a human woman who gave birth to his half-breed successor, Wataru. Making himself see the coexistence and peace between the two races as nothing more than a farce for his son's sake, the Beetle Fangire breaks the peace by attacking humans to gather their Life Energy and reclaim his title, even stealing Kivat from Wataru. His actions of removing the law to restart past conflicts between the two races provide the motivation for Wataru to step up and truly claim his place as King of the Fangires. In the end, he is weakened by Decade's Kiva Arrow, then fatally wounded by a Dimension Kick/Darkness Moon Break combo. Revealing that he knew of the Fangire's true identity and intent before he dies, Tsukasa plays a final song for him before the man shatters.
(I know this guy's not really a monster but I thought he deverves a mention)


Kamata is actually the Paradoxa Undead, a Category King Undead who was brought into Ryuki's World by Narutaki in an experiment. Assuming the guise of sub-chief editor of the Atashi Journal, he appeared to support Tsukasa and company in their investigation of the scene of chief editor Reiko Momoi's murder to clear Natsumi Hikari, helping provide whatever info he could before he and Tsukasa are summoned to join the Kamen Rider Trial in the Mirror World. When Kamen Rider Imperer interferes in the fight, Abyss decides to take advantage of this turn of events to see Decade's abilities, overseeing the later fight between Decade and Knight before attempting to take them and Ryuki out. After learning that Shinji and Tsukasa believe Ren to be Reiko's murderer, Kamata decides to kill Ren so the truth about his involvement would not be revealed. However, once Ren acquires the Time Vent from Odin, Shinji uses it so he and Tsukasa can stop Reiko's murder, learning that Kamata is the culprit, using his Undead powers to kill Reiko to silence her as she uncovered his secret. Kamata, from the present, followed Shinji and Tsukasa after the Time Vent was used. He then merges with his past self then sends his Advent Monsters after Tsukasa before Shinji intervenes. After being defeated by Decade and Ryuki, with his deck shattered and his Advent Monsters destroyed, Kamata's true nature as an Undead is revealed by Narutaki who takes him back to his home: Blade's World where he works with Hajime in a plot to take over the world as the chairman of the BOARD Corporation. After helping Hajime steal the three other Rider Systems, Kamata helps him create the Joker Rouse Card. He is later defeated alongside the Joker by Decade and Blade.
Kamen Rider Abyss is unusual amongst the Kamen Riders in that he has two Contract Monsters: the Abysslasher, a humanoid shark that can fire high pressure streams of water, and the Abysshammer, a humanoid hammerhead shark. This is different from Kamen Rider Imperier, who has multiple Mirror Monsters at his beck and call only because they follow the true Contract Monster of Imperer: Gigazelle.
Kamen Rider Abyss' Final Vent enables Abysslasher and Abysshammer to merge into a single giant Shark with a chainsaw attachment (giving it the appearance of a sawshark when in use) and machine gun eyes. It is destroyed by Ryuki when Decade modified him with the Final Formrider card that enabled Ryuki to turn into Dragreder.
Unlike other Riders in the Mirror World, the loss of the Contract Monsters did not result to Abyss going into Blank Form.
The Paradoxa Undead is a Category King Undead able to create a blade-like wind from his forearm to kill his target. Normally, he is disguised as a man named Kamata. After being exposed in Ryuki's World, Kamata returns to Blade's World where he works with Hajime in a plot to take over the world as the chairman of the BOARD Corporation. After helping Hajime steal the three other Rider Systems, Kamata helps him create the Joker Rouse Card. He is later defeated alongside the Joker by Decade and Blade.
Momose is a student of Smart Brain High who is a member of the Lucky Clover: a group of top-ranked students who are actually Orphnoch. Momose's true form is that of the
Tiger Orphnoch, possessing the power to revive Orphnochs on the verge of death. After learning the truth behind Faiz, Momose decides to take over the school and sire new Orphnoch from the student body. However, the Riders intervene and the Tiger Orphnoch is killed by Decade's Decade Photon.
Taurus Ballista, the Buffalo Lord, is one of the stronger Lords, armed with the Supremacy Torreana. He has Ant Lords as minions.