The Horoscopes, alternatively called the Twelve Constellations of the Zodiac are Zodiarts that are completely evolved from their human selves, keeping their human bodies, unlike less-advanced Last One Zodiarts and are based on the constellations of the Western Astrological Zodiac.
They answer directly to Mitsuaki Gamou, who share his ideals and believe themselves as superior to all others and that they were chosen by the universe itself. This is likely a side effect to the addiction of Cosmic Energy as seen on regular Zodiarts. They are distinctive in that they wear heavy cloaks that cover nearly all of their body, which they discard when entering battle and can be used as protection against certain attacks.
Any Horoscopes can create side troops using Cosmic Energy called the Stardust Ninja Dustards, which dissolves into dust after they are defeated.
Originally starting out with four members as the series begins, the Horoscopes work within AGHS to give Zodiarts switches to various students in order to build their membership to twelve as part of Gamou's master plan. Though the Kamen Rider Club members were in dark about this intent, Kamen Rider Meteor reveals the truth to them during his formal introduction.
Horoscopes Zodiarts possess personal Switches that are different from regular Zodiarts Switches which are red and gold in color (although, certain conditions must be met to fully awaken the switch, such as surpassing Last One stage or, like Ran's Pisces switch, must meet the condition that Pisces willingly pressed her switch), are marked with their respective astrology signs, and are oblong-shaped with no bottoms.
A Horoscopes' most powerful ability is to undergo a transformation called Supernova to either assume a more powerful form or gain a new technique. The Supernova can only be used for a limited period of time as overuse in a short period of time will cause the Zodiarts to be destroyed.
Once all twelve Horoscopes are gathered, Sagittarius will cause a mysterious event called the Day of Awakening where he uses all twelve Horoscopes Switches and the atmospheric vortex above the school known as The Hole to reach the Presenter(s), a mysterious sentient will of the universe that calls out to humanity, by conjuring the real Dark Nebula. But in the process, the Dark Nebula's gravitational pull would suck in all of Japan.
The membership had finally been risen to twelve, but there are now no Horoscopes active as they were all either defeated/killed at the hands of the Kamen Riders, or by other members of the Horoscopes.
Thanks to Libra faking defection, Sagittarius had all 12 Horoscopes Switches after getting Pisces' Switch. However, it was thanks to Kengo Utahoshi releasing a burst of Cosmic Energy from the Core Switch in his body that disrupted the Day of Awakening.
Members
Mitsuaki Gamou/Sagittarius
The leader of the Horoscopes, is the first to appear in the show and is the most powerful Zodiarts. Though appearing as a human for most of the series, he unveils his Horoscopes form around Virgo's death. He is currently deceased, having dissolved into Cosmic Energy as a side-effect of his evolution.
Kouhei Hayami/Libra
The second Horoscopes member to be seen on the show. He was the one that recruited most of the Horoscopes members into their fold. He once, willingly, allowed Leo to use his Switch, allowing two of his Dustards to evolve into Leo Dustards. He is currently deceased, after taking a hit from Fourze meant for Sagittarius. The hit then sent him flying backwards into an open rift into the Dark Nebula, where he is vaporized by the dimension's lightning.
Kou Tasugami/Leo
The third Horoscopes member to be seen on the show, having a deep connection to Sagittarius, and is fanatically loyal to him. He is currently deceased, as he dissolved into Cosmic Energy after his defeat by Kamen Rider Meteor.
Kuniteru Emoto/Virgo
The fourth Horoscopes to be seen on the show, and was Sagittarius's right hand. He was killed at the hands of Leo while fighting him and Libra when Sagittarius learns of Virgo's hidden agenda against him.
Sarina Sonoda/Scorpion
Sarina Sonoda she's the fifth Horoscopes member to be seen on the show. She is currently banished into the Dark Nebula by her fellow Horoscopes member, Virgo. Her Switch was later used by Leo. It is unknown where her current whereabouts is after Virgo told the Kamen Rider Club about sending them to the M-BUS and confirming the exact state of the Dark Nebula.
Natsuji Kijima/Cancer
The sixth Horoscopes member to be seen in the entirety of series and the fourth member on the show. This Horoscopes was once the Pegasus Zodiarts but has since evolved into the Cancer Zodiarts.
Cancer was banished into the Dark Nebula when Libra disguised him as a rather nosy detective from an earlier episode. Presenting Virgo with the Detective!Cancer, Libra was able to fool Virgo into sentancing him to eternal imprisonment within the Dark Nebula to protect Sagittarius' plans for Amanogawa. After his banishment, the Cancer Switch falls into the possession of Libra, though Libra does not use the switch. Cancer does, however reappear - though not as it's original holder. Through Leo's unique powers, he is able to transform into Cancer.
Once Virgo is exposed as being Tachibana, Virgo reveals to the Kamen Rider Club that he did not banish Cancer to the Dark Nebula, but rather he has him held in suspended animation on the M-BUS space station. We are not privy to Cancer's whereabouts at the end of the series. He is not seen again, and we can either assume he made it back safely - or that after Tachibana's death Cancer remained at the M-BUS space station in suspended animation.
Tatsumori Yamada/Aries
The seventh Horoscopes member, which evolved from the Lepus Zodiarts when he transfered from AGHS to Subaruboshi High School. He actually had no interest of joining the Horoscopes despite being one of Libra's prospects, but only wanting to rule over Subaruboshi High School. He's currently in a coma after his fight with Fourze in his newly obtained Cosmic States. His Switch was later used by Leo.
Tojiro Goto/Capricorn
The eighth Horoscopes member, which was found by Libra using his Eye of Laplace, showing his potential to evolve past the Lyra sign. He is currently living a normal teenage life, having his memories of being Capricorn erased after his defeat at Fourze's hands. His Switch was later used by Leo.
Erin Suda/Aquarius
The ninth member, as well as the second member that has a female body. She was found by Libra using his Eye of Laplace. She is currently living a normal teenage life, having her memories of being a Horoscopes removed from her after her defeat at Fourze's hands. Her Switch was later used by both Leo and Sagittarius.
Yuta Sugira/Taurus
The tenth member, who was found by Libra using the Eye of Laplace during the Astronaut Selection Exam. He was banished into the Dark Nebula after he attacked Virgo to protect his new friends. It is unknown where his current whereabouts is after Virgo told the Kamen Rider Club about sending them to the M-BUS and confirming the exact state of the Dark Nebula.
Dark Yuki/Gemini
The eleventh member, who was found by Libra using the Eye of Laplace under the command of Sagittarius. Gemini is actually the embodiment of Yuki's inner darkness that became a separate entity, thanks to the particular nature of evolution concerning people who have the potential to become a Gemini Zodiarts. After being weakened when "Light" Yuki took back dominance, Gemini is exorcised by Fourze in Cosmic States. Her Switch was later used by Leo.
Ran Kuroki/Pisces
The twelfth and final member, who was found by Libra using the Eye of Laplace. Pisces had previously appeared during the Musca Zodiarts incident, where she instigated most of the things that went wrong in that arc. Unlike the others, Pisces is not joining Sagittarius' fold, and is in fact rebelling to not allow the Day of Awakening to be realized. However, Libra and Leo used her very nature to protect the former Musca Zodiarts to get herself out in the open and took Pisces' Switch from her after beating her.
Musical Themes
The Horoscopes have many themes, two reoccurring scores being "Horoscopes Switch" and "Power of the Horoscopes".