The Fangires are the story's adaptation of the fictional Vampire creature of European folklore (their name being a combination of Fang and Vampire). These creatures are Stain Glass monsters that feed and sustain themselves by consuming the life force of humans, leaving their fallen prey in a transparent glass like state. Each Fangire belongs to a specific class based on the animal form it takes, and each Fangire has a true name. A Fangire can live up to three times longer than a human, barring being killed. Upon a natural death, the monster's physical body shatters like glass and collapses in a heap. The Fangire Race can assume human forms to hide within everyday society to feed in secret, revealing their forms when their skin takes a uniquely variant stained glass pattern. The Fangire Clans are the most feared and repected monsters in the history of the monster race. Not only that, they are ranked as SS-Class monsters that only S-Class monsters can beat them.
The members of the Fangire Race destroyed by Kiva have their bodies destroyed, with their souls sealed within the Castle Doran's body before they can escape and eventually reform into a new body or clump with other Fangires' souls to form a giant aura monster called a Sabbat. IXA, Kiva in Emperor Form, and Saga are able to destroy both body and soul. The Fangire Race is most prosperous of the 13 Demon Races, which also played a role in the near-extermination of the Wolfen, Merman, and Franken races. Not only that, they once tried to rule the Yuki-onna race led by Dark Kiva to conquered their home, turning it as a base and using them as livestock. But how they were driven off was unknown. Halfbreed Fangires like Wataru are known as Fandiri. They are mostly human but will show Fangire markings at times. It is revealed that vampires were more trusting of Fangires even above the other Mazoku and they are the only ones they respect as allies and friends since they are both vampire related. According to Maya on chapter 43, Maya's husband and Issa were good friends since Moka or even the Kurenai Brothers were ever born. The vampires also helped them on their war with the Legendorga Clan, earning them their respect.
The Wolfens of Kiva are based off of Lycanthropes, specifically Werewolves or Wolf-men of western (primarily European) myth and folklore. The name of the creatures are quite likely being the combination of the term Wolf-Man or Wolf-Men, or as well possibly being a reference to Whitley Strieber's horror novel (which was adapted to film in 1981), "The Wolfen". The last and only shown pack of Wolfens in the series was personally exterminated by the Fangire, Rook. The Wolfen possess heightened senses and use their claws to rip out human souls from their body before eating them. They also have a feral fighting style and are one of the fastest of the Demon Races. To werewolves, Wolfens are legendary since their ferocity earned them a place in the ranks of the Mazoku. Their presence alone made the werewolves feel pretty meek. Like they are being in the presence of an Alpha wolf shown on chapter 10 when Jiro had no problem silencing Gin by a single glare after he took the camera away from him.
The Mermen are primarily based off of the Gill-man monster of the classic 1954 American horror film, "The Creature from The Black Lagoon". However the name of the creatures is also generally what is used to refer to the male counterpart of the mythical sea dwelling Mermaid which the Swimming Club even Tamao were head over heels in love with Ramon after he shows his true form in chapter 48. The Mermen were, much like the other monster races, wiped out by the Fangires. Mermen can manipulate water and have keen senses, making them excellent marksmen. It is unknown how the Mermen feed off the human souls like the other Demon Races.
The Frankens are based off of Frankenstein's Monster of the classic horror novel by Mary Shelley, "Frankenstein; or, The Modern Prometheus". The name of the creatures quite obviously being a shortening of the Dr. character's name, Frankenstein. Much like the other races, the Franken were wiped out by the Fangires for unknown reasons. The Franken are physically the strongest of the Demon Races possessing unbeatable strength. Like other Demon Races, the Franken can feed off souls. They do this by grabbing a person and draining the soul out through an electric charge.
The Kivats are a based off of Vampire bats. Although not Humanoid in shape, Kivats posses the most coveted power amongst the monster races. Unlike the other races, the Fangires seem to allow the Kivats to stay alive, but likely in exchange for their service. The Kivats possess Demon Imperial Powers which they can instill into other lifeforms usually through their Active Fangs or other means. Some of them possess additional powers that is different amongst them. They also possess personas on their faces that are tradition in their families.
The Dorans were a race of large dragons. Although being listed as one of the 13, little information has been provided on them. Thus it is unknown if they died out, were exterminated by the Fangires, or if any more exist and roam free. The Doran feature the ability to seal souls and feed on its life energy. They can also breathe fire and fly like most dragons. They also feature abilities different amongst them.
The Legendorgas were featured only in the film installment of the series. Unlike the other monster races being derived from one western pop culture icon monster each, they are individually derived from many different monsters of European myth and folklore. In the mainstream of the film, they appear to have power and abilities that exceed that of the average Fangire. Despite their film only appearance, their canonical existence has been confirmed by being officially listed as one of the 13. But it is not known if they are still alive or if they've been driven into extinction by the Fangires. According to Maya on chapter 40, they have the ability to convert humans, lesser monsters, and even Demon Races into their kind. They also looked down on low class monsters as trash to them, thinking that they are superior to them because of their ranks as Mazoku.
The Takeshi Oni Clan was a nomadic clan of Mazoku who were the first to use music as a weapon. They called it Pure Sound. They respected nature and also protected humans from their enemies, the Makamou Clan. Both clans were in a war at the moment. The Takeshi Oni Clan was also secretive and hard to track down. Unlike most Mazoku, they didn't see humans as food. They even raised families with humans and their descendants, despite being mostly human, could still carry the Mazoku blood in them and possess the potential to become Oni.
An assortment of monstrous creatures that usually dwell in the rural areas and consumed human beings as food. In contrast to the humanoid adversaries in other Kamen Rider series, most of the Makamou were usually gigantic monsters of the kaiju genre whose designs and names were based on the Yōkai from Japanese folklores. Furthermore, the Makamou are aided by a mysterious pair of humans. The name Makamou can be translated "Demonized Spirits of the Countryside".
The Sagarks are an ancient Demon Race of UFO-like cobra monsters, created by the Fangire in ancient times. Like the Kivats, they wield Demon Imperial Power and can transmit it to other races in order to transform. Sagarks do not require biting the user, but instead synthesize a belt from their sides. They, also like Kivats, can utilize Wake Up Fuestles. Many come in different sizes, the largest being the enormous Mother Sagark. As they were created in olden times, Sagarks only speak in the dead Fangire tongue.