Metalgelas is a Mirror Monster that was originally contracted to Kamen Rider Gai but was later contracted to Kamen Rider Ouja. This makes him one of 5 Mirror Monsters that had more than 1 contractor. Together with Venosnaker and Evildiver, it became one of the building blocks of the composite Monster Genocider.
Metalgelas initially appears as Gai's Contract Monster but after Jun is killed, Asakura contracts him. Metalgelas resembles a humanoid rhinoceros and can reach extremely high speeds. He also has tough skin that can crush a moving car.
Metalgelas originally assisted Jun in both destroying monsters and attacking other Riders. However, once Jun died at the hands of Asakura, Metalgelas attacked many people who it believed resembled Asakura (as it was trying to avenge its contractor). However, Asakura was able to successfully contract Metalgelas and after Asakura obtained Evildiver also, it was able to merge Metalgelas with Evildiver and Venosnaker to create Genocider.
Metalgelas began to once again turn on Asakura after he neglected to feed Metalgelas life-force Energy from Mirror Monsters. In defiance, he attacked Asakura once before finally being fed just before Asakura completely breached his contract.
As Asakura did not die at the hands of a Rider, it is unknown what happened to Metalgelas. However, the End implies that because the Rider War was made nonexistent, that Metalgelas was also made nonexistent.
Kamen Rider Ryuki: Episode Final
Metalgelas makes a brief appearance in the film. He appeared when Asakura used his Unite Vent card to combine his Metalgelas with Evildiver and Venosnaker into Genocider. Metalgelas also destroyed alongside with Evildiver and Venosnaker when hitted by Kamen Rider Ryuga's Final Vent.
OOO, Den-O, All Riders: Let's Go Kamen Riders
Metalgelas was also one of the kaijin who gathers at Eiji Hino, Kotaro Nogami and Ankh/Momotaros's public execution. But however, when all Kamen Riders return to exist, Metalgelas tries to attack them although it is foiled by Kamen Rider Black RX.