Dark Kamen Riders 2.0

Updated from original 2010 post...
Here is a fan drawing of the Dark Riders in the Alternative World. There is a lot of Kamen Riders that rival the lead one, many evil, but here are the ones that are considered the Dark Kamen Riders and some other evil ones.

Shadow Moon
In Kamen Rider Black RX, Nobuhiko Akizuki, who had been abducted by Gorgom to become one of their new kings. Shadow Moon is both the stepbrother and best friend to Kohtaro Minami, who had been abducted alongside Nobuhiko but managed to escape.

Kamen Rider Ryuuga
Same thing with the last one, above is Onyx is from KRDK. He also appeared in Kamen Rider Ryuki: Episode Final, I have expressed how much I don't like his design since it is just a repaint of Ryuki. He later appeared in Kamen Rider Decade in the Negative World, he murdered and assumed the guise of the World of Negatives counterpart of Hirohiko Sato. He is part of Kamen Rider Diend Complete Form.

Pseudo-Rider Alternative
Pseudo-Rider Alternative is a Dark Rider who assumed the guise of the World of Negatives counterpart of Tanaka, Natsumi's high school teacher. Though ordered by Otoya to keep an eye on Natsumi rather than kill her, he intentionally decides to attack Natsumi, resulting in Dark Kiva beating him merciless. Alternative is then later destroyed by Diend. Although referred to as Alternative, the suit is actually that of Alternative Zero. Kamen Rider Alternative and Alternative Zero were not technically Dark Riders in Kamen Rider Ryuki.

Kamen Rider Ohja
Even though not officially a Dark Rider, I consider him so because he is just pure evil and even though a lot of Kamen Riders go against the main one (Knight for example), Ohja is nonredeemable.

Kamen Rider Orga
In the alternative world of Kamen Rider 555: Paradise Lost, there were two riders. The Orga gear was given to Yuji Kiba after he had been fooled into thinking he had been betrayed by the humans. He appeared in the Negative World in Kamen Rider Decade. Orga is part of Kamen Rider Diend Complete Form. He who murdered and assumed the guise of the World of Negatives counterpart of Kenji Sakata.

Kamen Rider Dark Kabuto
In the production of the Dark Kabuto costume, the production team took a Kamen Rider Caucasus costume and repainted it black with red circuitry details. Souji Kusakabe was originally a human infant used in Negishi's experiments in hopes of turning all of humanity into Natives. He appeared in the Negative World in Kamen Rider Decade. He murdered and assumed the guise of the World of Negatives counterpart of Kazuyoshi Aoyagi.

Kamen Rider Nega Den-O
He appears in Kamen Rider Den-O & Kiva: Climax Deka. Negataros and a select few Imagin are stray Imagin who evaded the fate of the other evil Imagin when Kai ceased to exist in the final episode. Negataros has to deal with both the DenLiner Police lead by Ryotaro Nogami, an inside man in the form of Yuto Sakurai, further worsened by the brief meddling of Wataru Kurenai.

Kamen Rider Dark Kiva
In the Kiva Series, the Checkmate Four's original King is referred to as the Dark Kiva, the lord of all Fangires and the original Kiva who was a threat to humanity in his time. However, Dark Kiva does not require the use of a being such as Tatsulot to achieve this form, and has no chains on his body as Kiva needs to control his power. In the Negative World of Kamen Rider Decade series, Otoya Kurenai is a young man who leads the Dark Riders, using his connection to Kivat-bat the 2nd to become Kamen Rider Dark Kiva.

Kamen Rider G4
He appears in Kamen Rider Agito The Movie: Project G4 in 2001 and is part of Kamen Rider Diend's Complete Form. The G4 suit then began to malfunction, ultimately killing Mizuki. Beginning to move on its own, the suit attempted to rise again to continue the fight, forcing Hikawa to destroy it with the Scorpion.

Kamen Rider Gaoh
Appears in Kamen Rider Den-O: I'm Born!, he is a king of bandits from the Warring States era and leads a troop of Imagin with which he plans on using to take over the DenLiner. On his journey, he also obtains Yukimura's help in unearthing the GaohLiner.

Kamen Rider Arc
He appeared in the movie Kamen Rider Kiva The Movie: King of the Castle in the Demon World. Arc appears part of Kamen Rider Diend Complete Form. Arc's name comes from a Japanese word for "evil."

Kamen Rider Core

Kamen Rider is the main antagonist in the final portion (Movie War Core) of the film, Kamen Rider × Kamen Rider OOO & W featuring Skull: Movie War Core. Kamen Rider Core is purely an energy-based Rider, he was created from the Sasori, Kani, and Ebi Core Medals, which appeared to house his consciousness, merging with the Memory Memory which held the memories of all of the older Showa Era Kamen Riders and the True Gaia Memory, thanks to the Pteradon Yummy gathering them.

Kamen Rider Eternal
Jun Kazu is the main antagonist of Kamen Rider W 's last story arc. He is a researcher of Foundation X wearing a lab coat at almost all times. He also appeared in Kamen Rider W Returns: Eternal, a prequel to the main TV series.\

Kamen Rider Jam/Mars

Kougane is Kamen Rider Mars who is the main antagonist in Kamen Rider Gaim: Great Soccer Battle! Golden Fruits Cup!. It later returns in the series finale as the female Kamen Rider Jam, serving as the series' final villain.

Another Riders

Kamen Rider Zi-O revealed evil Kamen Riders such as Another Riders. They are a group of monstrous Kamen Riders whose powers are derived from their original counterparts.

Let me know if I'm missing any.